DESCRIPTION Method and System for optimizing power output of a wind tur- bine with yaw misalignment Field of invention The present invention relates to a system comprising a wind turbine and a first control device which controls the wind turbine based on a yaw misalignment. The present invention also relates to a wind farm comprising the system, wherein a plurality of adjacent wind turbines is provided, and to a method of controlling a wind turbine. In a conventional wind farm, wake steering strategies by ad- justing wake effects between the wind turbines are performed to increase the annual energy production (AEP) of the wind farm. For example, an upstream wind turbine is set into a yawed condition, i.e., with a yaw misalignment, where a rota- tional axis of a hub is not fully aligned with the wind di- rection so as to deflect a wake produced by the upstream wind turbine away from a downstream wind turbine. By minimizing the wake effects, the power output of the wind farm is in- creased. The yawed conditions can therefore represent a de- sired yaw misalignment of the respective wind turbines to op- erate a wind farm controller in an optimum condition. If all wind turbines of the wind farm would be properly aligned in the wind direction, there would be no yaw misalignment, but the entire performance of the wind farm controller will be degraded due to the wake effects. The wake steering strate- gies require a proper operation of a wind turbine generator (WTG) under yawed conditions, where the wind turbines are not necessarily aligned in the wind direction. In the conventional control, the wind turbine is not subject- ed to any misalignment and adjusted based on aerodynamic properties and other characteristics of the wind turbine, such as mechanical and electrical efficiencies, rotor size, etc. In particular, a target pitch angle β is selected from a lookup-table in an open loop control as that angle which pro- vides the maximum power coefficient cp for a given, incoming wind speed U and a given rotor speed (=hub rotational speed) ωrot, i.e. the target pitch angle β is determined by a func- tion of the wind speed U and the rotor speed ωrot. A target torque TCtrl of the generator is also determined as a func- tion that depends on the rotor speed ωrot and on other char- acteristics of the wind turbine. Following both pitch and torque control laws, the operational point of the wind turbine converges to an optimum, whenever all assumptions are met. It is the aim of the controller to perform as optimum as possible under a rated power. The con- ventional controller is adjusted based on a model of the wind turbine which is aligned with the wind direction, and the conventional controller is designed for an optimum operation- al point under normal operation. However, if some wind tur- bines (for example in a wind farm) have a (desired) yaw misa- lignment, for example to avoid wake effects, the wind turbine does not operate in an optimum operational point. The wind turbine can also be subjected to a (desired) yaw misalignment due to reasons other than avoiding wake effects. Summary of the Invention It is the object of the present invention to provide a system comprising a wind turbine and a first control device which controls the wind turbine, which system can improve the annu- al energy production (AEP). It is also the object of the pre- sent invention to provide a wind farm comprising the system, where a plurality of adjacent wind turbines is provided, and a method of controlling a wind turbine, which can improve the annual energy production (AEP). This object is achieved by the subject matters according to the independent claims. The present invention is further de- veloped as set forth in the dependent claims. According to a first aspect of the invention, a system com- prising a wind turbine and a first control device. The wind turbine comprises a tower; a nacelle being mounted rotatable relative to the tower around a yaw axis so that the nacelle is orientated in an actual yaw angle; a hub being mounted ro- tatable relative to the nacelle around a rotational axis; at least one blade mounted to the hub; and a generator arranged in the nacelle and comprising an electromagnetic rotor con- nected to the hub, wherein the generator is configured to convert a rotational energy from the electromagnetic rotor into an electrical energy. The first control device is con- figured to acquire a yaw misalignment of the wind turbine, which yaw misalignment is a difference between the actual yaw angle and a wind direction at the wind turbine; and to deter- mine at least one of a target pitch angle of the blade and a target torque of the generator based on the yaw misalignment to optimize a power output from the wind turbine. Advantageously, the WTG is optimally operated in yawed condi- tions, loses only a minimum of energy yield in yawed condi- tions, makes the wind turbine control under wake steering more efficient and achieves higher wind farm energy yield. The performance of the wind farm will be increased by this improved WTG control with better and efficient performance at substantially no extra costs. For example, the first control device can be upgraded or retrofitted in existing wind tur- bines or wind farms. In an embodiment, the first control device is configured to determine the at least one of the target pitch angle of the blade and the target torque of the generator by calculating the target pitch angle of the blade as a function of an in- coming wind speed, a rotor speed of the hub and the yaw misa- lignment, and by calculating the target torque as a function of the rotor speed and the yaw misalignment. Compared to a conventional control, which disregards any yaw misalignment compensation, the present invention takes a change of the aerodynamic behaviour in a misaligned operation of the hub into account. The efficiency of the wind turbine is thus optimized. The pitch angle can be controlled in an open loop by taking the yaw misalignment into account, as the aerodynamic behaviour of the hub is different than in a nor- mal aligned operation. In an embodiment, the first control device is configured to determine the at least one of the target pitch angle of the blade and the target torque of the generator by considering a lookup-table, in which a relation between the incoming wind speed, the rotor speed of the hub and the yaw misalignment, and/or a relation between the target torque and the rotor speed and the yaw misalignment are stored. In an embodiment, the first control device is either inte- grated in the wind turbine or a remote control device. Pref- erably, the remote control device is connected to the wind turbine in a wireless manner. In an embodiment, the first control device is configured to determine the at least one of the target pitch angle of the blade and the target torque of the generator such that a tip- speed ratio is set, at which a power coefficient of the wind turbine becomes an optimum at the given yaw misalignment. According to a second aspect of the invention, a wind farm is provided. The wind farm comprises the system, wherein a plu- rality of adjacent wind turbines is provided. The wind farm comprises a second control device which is configured to de- termine the respective actual yaw angles of the plurality of adjacent wind turbines based on wake conditions between the plurality of adjacent wind turbines. In an embodiment, the second control device which is config- ured to determine the respective actual yaw angles of the plurality of adjacent wind turbines based on the wake condi- tions between the plurality of adjacent wind turbines such that a total power output of the wind farm becomes an opti- mum. In an embodiment, the first and second control devices are either integrated as a unit or separate control devices. According to a third aspect of the invention, a method of controlling a wind turbine is provided. The wind turbine com- prises a tower; a nacelle being mounted rotatable relative to the tower around a yaw axis so that the nacelle is orientated in an actual yaw angle; a hub being mounted rotatable rela- tive to the nacelle around a rotational axis; at least one blade mounted to the hub; and a generator arranged in the na- celle and comprising an electromagnetic rotor connected to the hub, wherein the generator is configured to convert a ro- tational energy from the electromagnetic rotor into an elec- trical energy. The method comprises steps of acquiring a yaw misalignment of the wind turbine, which yaw misalignment is a difference between the actual yaw angle and a wind direction at the wind turbine; and determining at least one of a target pitch angle of the blade and a target torque of the generator based on the yaw misalignment to optimize a power output from the wind turbine. In an embodiment, the at least one of the target pitch angle of the blade and the target torque of the generator is deter- mined by calculating the target pitch angle of the blade as a function of an incoming wind speed, a rotor speed of the hub and the yaw misalignment, and by calculating the target torque as a function of the rotor speed and the yaw misalign- ment. In an embodiment, the at least one of the target pitch angle of the blade and the target torque of the generator is deter- mined by considering a lookup-table, in which a relation be- tween the incoming wind speed, the rotor speed of the hub and the yaw misalignment, and a relation between the target torque and the rotor speed and the yaw misalignment are stored. In an embodiment, the at least one of the target pitch angle of the blade and the target torque of the generator is deter- mined such that a tip-speed ratio is set, at which a power coefficient of the wind turbine becomes an optimum at the given yaw misalignment. In an embodiment, a plurality of adjacent wind turbines is provided, and the respective actual yaw angles of the plural- ity of adjacent wind turbines are controlled based on wake conditions between the plurality of adjacent wind turbines. In an embodiment, the respective actual yaw angles of the plurality of adjacent wind turbines are controlled based on the wake conditions between the plurality of adjacent wind turbines such that a total power output of the wind farm be- comes an optimum. It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with ref- erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti- fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea- tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application. Brief Description of the Drawings The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited. Fig. 1 shows a wind turbine and the different elements thereof; Fig. 2 shows a schematic configuration of a wind- turbine-generator-system (WTG system) according to an embodiment; and Fig. 3 shows operational points according to an embodi- ment vis-à-vis the prior art on a graph of an aerodynamic power coefficient cp against a tip- speed ratio λ. Detailed Description The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele- ments are provided with the same reference signs. Fig. 1 shows a wind turbine 1. The wind turbine 1 comprises a nacelle 3 and a tower 2. The nacelle 3 is mounted at the top of the tower 2. The nacelle 3 is mounted rotatable relative to the tower 2 around a yaw axis 10 by means of a yaw bear- ing. The wind turbine 1 also comprises a hub 4 with three rotor blades 6 (of which two rotor blades 6 are depicted in Fig. 1). The hub 4 is mounted rotatable relative to the nacelle 3 by means of a main bearing 7. The hub 4 is mounted rotatable about a rotor axis of rotation 8. The wind turbine 1 furthermore comprises a generator 5. The generator 5 in turn comprises an electromagnetic rotor con- necting the generator 5 with the hub 4. If the hub 4 is con- nected directly to the generator 5, the wind turbine 1 is re- ferred to as a gearless, direct-driven wind turbine. Such a generator 5 is referred as direct drive generator 5. As an alternative, the hub 4 may also be connected to the generator 5 via a gear box 9 (see Fig. 2). This type of wind turbine 1 is referred to as a geared wind turbine. The present inven- tion is suitable for both types of wind turbines 1. The generator 5 is accommodated within the nacelle 3. The generator 5 is arranged and prepared for converting the rota- tional energy from the hub 4 into electrical energy in the shape of an AC power. An operation of the wind turbine 1 is controlled by a first control device (not shown). The first control device can be either integrated in the wind turbine 1 or a remote control device. For example, the first control device as a remote control device can be configured to control a plurality of adjacent wind turbines 1 of a wind farm. The remote control device is preferably connected to the wind turbines 1 of the wind farm in a wireless manner. Fig. 2 shows a schematic configuration of a wind-turbine- generator-system (WTG) according to an embodiment. The wind having a wind speed U drives the blades 6 and the hub 4 which is connected to the electromagnetic rotor of the generator 5 via a gearbox 9. The hub 4 rotates with a rotor speed ωrot. The blades 6 are adjusted to have a (preferably common) tar- get pitch angle β which is an orientation of the blades 6 around the longitudinal axes of the respective blades 6. The wind rotates the hub 4 by an aerodynamic power Pwareo and an aerodynamic torque Taero. The first control device determines a set point of the target torque TCtrl of the generator 5. A mechanical efficiency of a system consisting of the blades 6, the hub 4 and the gearbox 9 is designated by reference sign ηmech. The first control device is configured to acquire a yaw misa- lignment γ of the wind turbine 1, which yaw misalignment γ is a difference between an actual yaw angle of the nacelle 3 and the hub 4, and a wind direction at the wind turbine 1. The first control device can either be configured to directly measure the wind direction or to receive information about the wind direction from a remote device (for example from a second control device for controlling a plurality of wind turbines 1 in a wind farm). The first control device is further configured to determine at least one of a target pitch angle β of the blade 6 and the target torque TCtrl of the generator 5 based on the yaw misa- lignment γ to optimize a power output from the wind turbine 1. For example, the first control device can be configured to determine the at least one of the target pitch angle β of the blade 6 and the target torque TCtrl of the generator 5 based on the yaw misalignment γ to maximize a power output from the wind turbine 1. The power output of the wind turbine 1 is the output amount of electrical energy per unit time. In detail, the first control device is configured to determine the at least one of the target pitch angle β of the blade 6 and the target torque TCtrl of the generator 5 by calculating the target pitch angle β of the blade 6 as a function of the in- coming wind speed U, the rotor speed ωrot of the bub 4 and the yaw misalignment γ, and by calculating the target torque TCtrl as a function of the rotor speed ωrot and the yaw misa- lignment γ. For example, the target torque TCtrl can be cal- culated like TCtrl = f(ωrot) * X(γ). Preferably, the first control device can be configured to de- termine the at least one of the target pitch angle β of the blade 6 and the target torque TCtrl of the generator 5 by considering a lookup-table, in which a relation between the incoming wind speed U, the rotor speed ωrot of the hub 4 and the yaw misalignment γ, and/or a relation between the target torque TCtrl and the rotor speed ωrot and the yaw misalign- ment γ are stored. Fig. 3 shows operational points 21, 210 according to an em- bodiment vis-à-vis the prior art on a graphs 20, 200, which represent an aerodynamic power coefficient cp against a tip- speed ratio (TSR) λ, respectively. The tip-speed ratio λ is a ratio between a tangential speed of the tip of a blade 6 and the actual wind speed U. Both graphs 20, 200 are plotted at a given yaw misalignment γ. Reference sign 200 designates a prior art graph, on which an operational point 210 is set, which does not consider the yaw misalignment γ. Reference sign 20 designates a graph according to an embodiment of the present invention, on which an operational point 21 is set, which considers the yaw misalignment γ. It turns out from Fig. 3 that the operational point 21 of the present invention is an optimum operational point, while the operational point 210 of the prior art is a sub-optimum operational point 21. In other words, the first control device is configured to de- termine the at least one of the target pitch angle β of the blade 6 and the target torque TCtrl of the generator 5 such that a tip-speed ratio λ is set, at which a power coefficient cp of the wind turbine 1 becomes an optimum at the given yaw misalignment γ. If the control algorithm would not take the yaw misalignment γ into account, the wind turbine 1 would converge to the sub- optimum operational point 210. However, by taking the yaw misalignment γ into account, the loss of efficiency is re- moved, and the wind turbine performance is optimized. In Fig. 3, the graphs 200, 210 of the aerodynamic power coef- ficient cp against the tip-speed ratio λ are the same for the prior art and the present invention; however, the graphs 200, 210 of the aerodynamic power coefficient cp against the tip- speed ratio λ of prior art and the present invention can be different from each other. The present invention is particularly useful in controlling the wind farm, where a plurality of adjacent wind turbines 1 is provided. Usually, a wind turbine 1 produces a wake effect to an adjacent, downstream wind turbine 1, which wake effect could impair the output power of the adjacent, downstream wind turbine 1 and thus the efficiency of the entire wind farm. In order to avoid such loss of efficiency, a second control device (not shown) is provided which minimizes the loss of efficiency by minimizing the wake effect. This is achieved by setting a yaw misalignment γ of a specific wind turbine 1 so as to deflect the wake away from other wind tur- bines 1. The term yaw misalignment γ here relates to a de- sired difference between the wind direction at the wind tur- bine 1 and the adjusted actual (and desired) yaw angle. On the one hand, the specific wind turbine 1 will have a reduced output power, but on the other hand, the wake effects to oth- er wind turbines 1 are minimized, which in turn improves the overall efficiency of the entire wind farm. The second control device is thus configured to determine the respective actual yaw angles of the plurality of adjacent wind turbines 1 based on wake conditions between the plurali- ty of adjacent wind turbines 1. Preferably, a total power output of the wind farm becomes an optimum. For example, the total power output of the wind farm is maximized. The first and second control devices can be either integrated as a unit or separate control devices. It should be noted that the term “comprising” does not ex- clude other elements or steps and “a” or “an” does not ex- clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con- strued as limiting the scope of the claims.