SYSTEM COMPRSING AN INDUSTRIAL ROBOT AND AN END EFFECTOR WITH POWER TOOL AND CHARGER Field of the invention The present disclosure generally relates to the field of industrial robots having end effectors for operating power tools. More specifically, the present disclosure relates to charging accumulator modules for powering such power tools. Background of the invention In manufacturing industry, industrial robots may be used to process objects by means of power tools. Such industrial robots are equipped with a so called end effector, which is a device attached to the end of the robot arm and designed to interact with the objects and/or surrounding environment. The end effector is usually referred to as the last link of the robot. The end effector may typically comprise the power tool and is mounted to the robot arm end portion. The end effector may also comprise some kind of (energy) accumulator module, such as a battery, to power operation of the power tool. The accumulator module needs to be regularly charged in order to power the power tool properly. The robot then typically unloads the empty accumulator module in a charger stand, and picks up another, charged, accumulator module from the stand. This process is cumbersome and time consuming and, thus, reduces the efficiency of the manufacturing process. Summary of the invention It would be advantageous to achieve a system overcoming, or at least alleviating, the above mentioned drawback. In particular, it would be desirable to enable a system facilitating charging of power tools operated by industrial robots. To better address one or more of these concerns, a system having the features defined in the independent claim is provided. Preferable embodiments are defined in the dependent claims. A system is provided comprising an industrial robot comprising a robot arm end portion and a power supply circuit arranged to supply current to the robot arm end portion. The system further comprises an end effector comprising a power tool and an (energy) accumulator module arranged to power operation of the power tool. The end effector is attached to the robot arm end portion. The end effector further comprises a charger arranged to draw current from the power supply circuit at the robot arm end portion and to supply charging current to the accumulator module. The inventors have realized that the current available at the robot arm end portion can be utilized to charge the accumulator module for the power tool. The current available from the power supply circuit of a standard industrial robot is typically not enough to provide the power tool with its peak operating current, but it may be used to continuously charge the accumulator module when needed. Hence, the accumulator module may provide the power tool with its peak power during operation, while the current from the industrial robot is utilized for recharging the accumulator module as needed. This reduces the need of replacing empty accumulator modules with charged ones, which saves time in the manufacturing process. Further, the need for a separate charging stand for accumulator modules is reduced, which saves resources in the manufacturing process. In conclusion, charging of the accumulator module for the power tool is facilitated and the efficiency of the manufacturing process is increased. For example, the system may be arranged such that the charging current (available from the power supply circuit of the industrial robot) is a fraction of a peak current consumed by the power tool during its operation, such as less than 1/3 ^rd of the peak current consumed by the power tool during its operation, such as less than 1/5 ^th of the peak current consumed by the power tool during its operation, such as around 1/10 ^th of the peak current consumed by the power tool during its operation. For example, the current available from the power supply circuit may be around 1-3A, while the peak current consumed by the power tool may be around 15A – 30A. Hence, the accumulator module may be utilized to provide the relatively high peak current needed for operating the tool, while the relatively low current available from the industrial robot is used for charging the accumulator module. For example, the capacity of the accumulator module may be adapted to provide an entire peak current consumption of the power tool when it is operated. Hence, the main power source for operating the power tool may be the accumulator module. According to an embodiment, the system may further comprise a control device configured to control the charging of the accumulator module by the charger. Hence, the control device may be utilized to manage the charging of the accumulator module in different ways, thereby enabling several advantages as will be described further below. The term “control device” is to be broadly interpreted as any means, distributed or centralized, configured to control the charging of the accumulator module. The control device may alternatively be referred to as a control unit, control system or control circuit. According to an embodiment, the control device may be configured to control the charging current to be below a (preset) current threshold being less than the total available current from the power supply circuit (of the industrial robot). Hence, part of the current from the power supply circuit may be utilized for other purposes, such as for a vison camera or a gripping tool installed at the industrial robot. For example, if the total available current from the industrial robot is around 2A, the charging current may be controlled to not exceed a current threshold of 0,7A. According to an embodiment, the current threshold may simply be preset, such as to a static value. For example, it may be preset by an operator setting up the system or preset factory wise. According to another embodiment, the current threshold may be based on one or more dynamic parameters (e.g. control signals), which enables a smarter control of the charging. For example, if a control system of the robot indicates that the robot and its power tool is not to be operated for a certain period of time, the current threshold can be increased, thereby speeding up the charging of the accumulator module. On the other hand, if more current is needed for operation of the industrial robot, the threshold may be lowered. According to an embodiment, the control device may be configured to control the charging such that the accumulator module is charged by the charger between operations performed by the power tool, and optionally also during operations performed by the power tool. According to embodiments, the control device may be arranged in the industrial robot and/or in the end effector. For example, the control device may be arranged in the accumulator module, in the power tool or in a holder of the end effector for holding the power tool. Alternatively, the control device may be arranged separate from, but in communication to, the industrial robot and the end effector. According to an embodiment, the control device may be configured to control the charging such that a charging level of the accumulator module is kept below a maximum charge capacity of the accumulator module, such as below 80% of the maximum charge capacity of the accumulator module, such as below 70% of the maximum charge capacity of the accumulator module, such as below 60% of the maximum charge capacity of the accumulator module. Having an upper charging limit increases the energy accumulator life time, and is particularly advantageous in the present system since the accumulator module may be connected to the charger for most of the time. According to an embodiment, the control device may be configured to control the charging such that a charging level of the accumulator module is kept above a minimum threshold, such as above 20% of the maximum charge capacity of the accumulator module, such as above 30% of the maximum charge capacity of the accumulator module, such as above 40% of the maximum charge capacity of the accumulator module. Avoiding the accumulator module getting fully depleted increases its life time and reduces the risk of interrupted operation of the power tool. As the accumulator module in the present system may be connected to the charger for most of the time and, in addition, time consuming operations of replacing an empty accumulator module can be avoided, a relatively small state of charge window can be used, which is beneficial for the accumulator module health. According to an embodiment, the control device may be configured to trigger a warning signal when the charging level of the accumulator module is too low and/or too high. This is to reduce the risk of the accumulator module getting depleted, which may interrupt the manufacturing process as the power tool cannot be operated, and/or the risk of the accumulator module being more charged than wanted, which may be unhealthy for the accumulator module. For example, a warning signal may be provided if the charging level of the accumulator module is below a certain threshold defining a lowest acceptable charging level, and/or above another certain threshold defining a highest acceptable charging level. According to an embodiment, the end effector may comprise a holder arranged to hold the power tool, the holder being attached to the robot arm end portion. This facilitates changing power tool, e.g. to another type of power tool. Hence, the power tool and holder may be separate parts. Alternatively, the power tool may be an integrated part of the end effector. Changing power tool may then include changing the whole end effector. According to embodiments, the charger may be arranged at the power tool, or elsewhere in the end effector, such as at the holder. According to an embodiment, the accumulator module may be attached and connected to the charger. According to an embodiment, the robot arm end portion may comprise an electrical contact (such as a socket) adapted to mate with an electrical contact of the end effector for electrically connecting the charger of the end effector with the power supply circuit of the industrial robot. It is noted that embodiments of the invention relates to all possible combinations of features recited in the claims. Brief description of the drawings The system will now be described in more detail in the following illustrative and non-limiting detailed description of embodiments, with reference to the appended drawings. Figure 1 shows a system according to an embodiment. Figure 2 shows a part of the system including the end effector and the robot arm end portion in more detail. All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted. Like reference numerals refer to like elements throughout the description. Detailed description of embodiments As illustrated in Figure 1, a system 1 according to an embodiment comprises an industrial robot 2 and an end effector 5 to be operated by the robot 2. The system 1 may be automated and capable of movement on at least one, but preferably two or more axes. For example, the system 1 may be adapted to be used for manufacturing, such as for assembling parts. The industrial robot 2 may comprise a robot arm 16, which may have one or more joints 17 for enabling movement of the robot arm 16 around one or more axes. A robot arm end portion 3 is provided at the distal end of the robot arm 16. The industrial robot 2 further comprises a power supply circuit 4 arranged to supply current to the robot arm end portion 3. The power supply circuit 4 may e.g. draw current from the mains. The power supply circuit 4 may comprise, or be connected to, a control device 9 for the industrial robot 2. The control device 9 may be configured to control the power supply circuit 4. For example, the industrial robot 2 may be stationary and may be arranged to be placed along an assembly line. The end effector 5 is attached to the robot arm end portion 3 and comprises a power tool 6 and an accumulator module 7 arranged to power operation of the power tool 6. The power tool 6 may e.g. be some kind of tightening tool, riveting tool, or drill. The accumulator module 7 may e.g. comprise a battery, a capacitor or any other kind of means for accumulating energy to be used for powering the tool 6. For example, the end effector 5 may comprise a holder 11 arranged to hold the power tool 6. The holder 11 may be the part of the end effector 5 that is attached to the robot arm end portion 3. The power tool 6 may be mounted to the holder 11. Alternatively, the holder 11 may be integrated with the power tool 6. For example, the accumulator module 7 may be attached to/comprised in the holder 11 as illustrated in Figure 1 and 2, or be attached to/comprised in the power tool 6 (not shown). The end effector 5 further comprises a charger 8 arranged to draw current from the power supply circuit 4 at the robot arm end portion 3 and to supply charging current to the accumulator module 7. An electrical connection may be provided between the power supply circuit 4 at the robot arm end portion 3 and the charger 8 in the end effector 5 so as to supply current there between for enabling charging. The charger 8 may comprise an electrical contact 15 adapted to mate with an electrical contact 14 of the accumulator module 7 (see Figure 2). During operation, current flows from the power supply circuit 4 to the end effector 5 and its’ charger 8, and then from the charger 8 to the accumulator module 7. Preferably, the accumulator module 7 may be connected (and attached) to the charger 8 during operation of the system 1, including operation of the power tool 6. The system 1 may further comprise a control device 10 configured to control the charging of the accumulator module 7 by the charger 8. The control device 10 may be arranged in the end effector 5, such as in the holder 11 as illustrated in Figure 1, or in the power tool 6 (not shown). Alternatively, the control device 10 may be arranged in the industrial robot 2, such as in (or in connection to) the control device 9 for the power supply circuit 4. Turning now to Figure 2, embodiments of the system 1 will be described in more detail. For example, the holder 11 may comprise a charger portion 18 including the charger 8 and arranged to hold the accumulator module 7. A cable 12 may provide the charger portion 18 with power from the power supply circuit 4 in the robot arm end portion 3. The cable 12 may be routed outside or inside (the latter not shown) the robot arm end portion 3 and the end effector 5. The end effector 5 may comprise circuitry for providing power from the accumulator module 7 to the power tool 6. For example, a cable 13 may provide the power tool 6 with power from the charger portion 18. According to an embodiment, the charging current for charging the accumulator module 7 may be just a fraction of the peak current that is consumed by the power tool 6 during its operation. The power tool 6 may typically require a peak current around 15-30A when operated while the charging current may be limited to around 0,5-3A. For example, the charging current may be less than 1/3 ^rd of the peak current consumed by the power tool 6 during its operation, such as less than 1/5 ^th of the peak current consumed by the power tool 6 during its operation. For example, the accumulator module 7 may have a capacity that can provide the power tool 6 with its peak current during operation. The accumulator module 7 may e.g. comprise a 18, 24, 36 or 48 V battery. According to an embodiment, the control device 9 may be configured to control the charging of the accumulator module 7 such that the charging current is kept below a (preset or dynamic) threshold being lower than the total available current from the power supply circuit 4. Hence, the charging of the power tool 6 may only utilize some of the available current from the industrial robot 2, thereby enabling some of the current available from the industrial robot 2 to be utilized for other purposes. The power tool 6 may be charged only between operations of the power tool 6, or simply continuously as long as the charging level of the accumulator module 7 is below a certain (preset) threshold. The control device 10 may be configured to control the charging level of the accumulator module 7 to be below a (preset) maximum threshold and optionally also above a (preset) minimum threshold. This may be referred to as the state of charge window of the accumulator module 7. The state of charge window may e.g. have a lower threshold comprised within a range of 20-40% of the maximum charge capacity. The upper threshold may be comprised within a range of 60- 80% of the maximum charge capacity. As the accumulator module 7 can be continuously charged in the end effector 5, the state of charge window may be kept relatively small, which is beneficial for the accumulator module 7 health. According to an embodiment, the control device 10 may be configured to provide a warning signal in response to the charging level of the accumulator module 7 falling below a minimum threshold and/or exceeding a maximum threshold. An operator of the system 1 may then be notified that charging of the accumulator module 7 has not been performed as excpected. The warning signal may e.g. be a visual signal and/or an audio signal. The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.