The present invention relates to a burner as- sembly comprising a nozzle engaged in a nozzle holder arranged at a distal end of a nozzle pipe having a first fluid conduit for delivering a first fluid to the nozzle, and a proximal end of the nozzle pipe be¬ ing in releasable connection with a connection block providing a first connection conduit in communication with said first fluid conduit. Prior art burners have found widespread use as domestic burners, primarily burning oil. However, maintenance and repair is often somewhat complicated. At regular intervals the burner assembly should be inspected to ensure safe and environmentally sound operation of the burner assembly. The inspection will often include a visual inspection of the nozzle and if necessary a replacement of the nozzle. Inspection of the nozzle is however troublesome as the nozzle is located in the furnace and the nozzle must be with¬ drawn for inspection. This means that the oil line to the nozzle must be disconnected, which often is labo¬ rious and entails a risk of leaks at reconnection. US Patent no. 2,760,564 discloses a fuel supply system for oil burners. This fuel supply system makes use of an adapter, having a bore and an air inlet passage and an oil inlet passage. A nozzle of the burner is threaded into one end of an air tube, and the other end of this tube is threaded into the adapter, which is bolted to a housing. It is rela¬ tively complicated to disconnect and withdraw the nozzle from the furnace for inspection. Further the fuel supply system is suited mainly for the specific burner. It is hence an object of the present invention to provide a universal burner assembly, which facili- tates mounting of the burner assembly to and disas¬ sembling of the burner assembly from a furnace. To achieve this object the burner assembly out¬ lined above is characterized in said burner assembly comprising connection means for releasable connection of the nozzle pipe and the connection block, said connection means being adapted to allow free rotation of the nozzle pipe with respect to the connection block. Hereby a quick-acting coupling is provided, whereby the first fluid conduit of the nozzle pipe may be connected to and disconnected from a fluid source in a very convenient and handy way. The fea¬ ture that the connection means is adapted to allow free rotation of the nozzle pipe with respect to the connection block renders the burner universal in that the orientation of the connection block is open to the liberty of choice. Hence the burner assembly may easily be fitted to a variety of furnaces irrespec¬ tively of the routing of supply tubes, as the connec¬ tion block of the burner assembly may be oriented ac- cordingly. The advantages of the burner assembly is even more pronounced according to an embodiment, wherein said nozzle pipe further comprises a second fluid conduit in . communication with a second connection conduit of the connection block. Hereby a quick- acting coupling is provided, with which it is possi¬ ble to disconnect the two conduits from the nozzle pipe in one operation, so the time needed is consid- erably reduced. Further there is no risk of errors with regard to confusion of the two conduits, as the first and second conduits automatically will be re¬ connected to the correct one of connection conduits of the connection block at reassembly. The first and second conduits of the nozzle pipe may be provided in any way considered expedient by the skilled person, such as a pair of parallel bores. According to an embodiment, however, the first and second fluid conduits in the nozzle pipe are con¬ centric, which for example can be achieved in a sim¬ ple and relatively inexpensive way with a pair of pipes, one inserted in the other. The releasable connection of the nozzle pipe to the connection block may be a simple press-fit. How¬ ever, to avoid any accidental disengagement of the nozzle pipe from the connection block, the burner as¬ sembly may further comprise securing means arranged at or near a side of the connection block being re- mote from the nozzle for retention of the nozzle pipe in the connection block. By arranging the securing means at or near a side of the connection block being remote from the nozzle, it is ensured that the secur¬ ing means is accessible to the service mechanic. In an embodiment, said previously mentioned connection means includes said proximal end of the nozzle pipe comprising a thread and being inserted in a through-going hole of the connection block to pro¬ ject from a back side of the connection block, and a securing means for engagement with said thread, such as a nut. Hereby a secure, but releasable connection is achieved, which is simple and hence inexpensive and further intuitive to the service mechanic. Fur- ther the nut will be very accessible for the service mechanic to manipulate, and hence the time and cost involved with service is low, and likewise the risk of injury. According to an alternative, said connection means includes the nozzle pipe comprising a radial hole at the proximal end thereof, and the nozzle pipe being inserted in a through-going hole of the connec¬ tion block to project from a back side of the connec- tion block, and a securing means for engagement with said radial hole, such as a cotter pin. This embodi¬ ment will also provide a secure, but releasable con¬ nection, which is simple and hence inexpensive and further intuitive to the service mechanic. Further the cotter pin will be very accessible for the ser¬ vice mechanic to manipulate, and hence the time and cost involved with service is low, and likewise the risk of injury. According to yet another alternative, said con- nection means includes the nozzle pipe comprising a circumferential groove at the proximal end thereof, and the nozzle pipe being inserted in a blind hole of the connection block, and a securing means for en¬ gagement with said groove, such as a securing pin. This embodiment will also provide a secure, but re- leasable connection, which is simple and hence inex¬ pensive and further intuitive to the service me¬ chanic. Further the securing pin will be very acces¬ sible for the service mechanic to manipulate, and hence the time and cost involved with service is low, and likewise the risk of injury. In an alternative to the nozzle pipe being made up of two pipes inserted one into the other, the noz- zle pipe is a unitary construction, such as an ex¬ truded pipe element, having parallel conduits therein. With this construction the number of joints is reduced, and the risk of leaks is considerably re- duced, and the joining together of the parts making up the burner assembly is facilitated, bringing down the cost of manufacture and maintenance. According to an embodiment, the nozzle pipe and the nozzle holder is a unitary construction, such as an extruded pipe element, which further reduces the number of joints, and the risk of leaks, and the joining together of the parts making up the burner assembly is facilitated, bringing down the cost of manufacture and maintenance. According to an embodiment, the burner assembly is a domestic-type burner assembly, such as a burner assembly having an output in the range of approxi¬ mately 1-100 kW. To reduce the risk of spillage and escape of fluid, the connection block may comprise a solenoid valve, to enable closing of the supply of fluids to the connection block, and hence close to the nozzle, whereby low emissions are obtained. The burner may be of any type, however accord- ing to an embodiment the burner is of a spray-type using an atomizing gas to atomize a liquid fuel. This type of burner enables a wide regulation of the out¬ put from the nozzle compared to the common type using pressurized fuel. With the common type of pressure atomizing burner the output can be regulated by a factor 2 in normally 2-3 steps, whereas a burner us¬ ing pressurized gas enables a stepless regulation by a factor of 5, and with a lower minimum output. According to another aspect, the invention re¬ gards a connection block for a burner, said connec¬ tion block comprising an opening adapted for releas- able engagement with a proximal end of a nozzle pipe, said connection block comprising a first connection conduit arranged to communicate with a corresponding first conduit in the nozzle pipe at insertion of the proximal end of the nozzle pipe in said opening in the connection block. To provide for easy and safe mounting and dis¬ mounting of the burner assembly and allow universal application thereof, the connection block is adapted for mutual, free rotation of the nozzle pipe in the opening. Hereby the connection block provides a quick-acting coupling, whereby the first fluid con¬ duit of the nozzle pipe may be connected to and dis¬ connected from a fluid source in a very convenient and handy way. The feature that the connection block is adapted for mutual, free rotation of the nozzle pipe in the opening renders the connection block uni¬ versal in that the orientation of the connection block is open to the liberty of choice. Hence the connection block may easily be fitted to a variety of furnaces irrespectively of the routing of supply tubes, as the connection block of the burner assembly may be oriented accordingly. According to an embodiment, the connection block further comprises a second connection conduit arranged to communicate with a corresponding second fluid conduit in the nozzle pipe at insertion of the nozzle pipe in the connection block. The opening in the connection block for inser¬ tion of the nozzle pipe may be a blind hole. Accord- ing to an embodiment, however, the opening is a through-going hole. The through-going hole enables the nozzle pipe to extend through the connection block and be secured therein by a suitable securing means, such as a nut threaded onto the projecting nozzle pipe end. In the following the invention will be de¬ scribed in further detail by way of example only, and with reference to the drawings, in which: Fig. 1 is a longitudinal section of a burner assembly according to the invention, Fig. 2 is an enlarged view of a detail II of Fig. 1, Fig. 3 is an enlarged view of a detail III of Fig. 1, Fig. 4 is an isometric view of the burner as¬ sembly of Fig. 1, lFig. 5 is a longitudinal section corresponding to Fig. 3 of an embodiment having a solenoid valve, Fig. 6 is a cross sectional view of the embodi¬ ment of Fig. 5, Fig. 7 is an enlarged view of detail of Fig. 3, Fig. 8 is an enlarged view corresponding to Fig. 7 of an alternative embodiment, Fig. 9 is an enlarged view corresponding to Fig. 7 and 8 of another alternative embodiment, Fig. 10 is a side view of an alternative burner assembly, Fig. 11 is a cross section according to line XI in Fig. 10, and Fig. 12 is a longitudinal section of a part of the burner assembly in Fig. 10. With reference to Fig. 1, 2 and 3 is shown a burner assembly 1 comprising a nozzle 2 connected to a nozzle holder 3. A distal end 4 of a nozzle pipe 5 is connected to the nozzle holder 3, whereas a proxi¬ mal end 7 of the nozzle pipe 5 is inserted in a con- nection block 6. A first fluid conduit 11 is arranged in the nozzle pipe 5 to lead a fluid from the connec¬ tion block 6 to the nozzle 2. The connection block 6 comprises a first connection conduit 9 in communica¬ tion with the first fluid conduit 11. A first tube 16 is coupled to the first connection conduit 9 for de¬ livering a first fluid, such as oil, from a first fluid source to the nozzle 2. In the shown embodi¬ ment, the nozzle pipe 5 further comprises a second fluid conduit 12 arranged to communicate with a sec- ond connection 13 in the connection block 6. A second tube 17 is coupled to the second connection conduit for delivering a second fluid, such as atomizing air, from a second fluid source to the nozzle 2. As best seen in the enlarged detail view of Fig. 3, the nozzle pipe 5 is inserted in the connec¬ tion block 6 to register openings 18, 19 of the first and second fluid conduits 11, 12 with the connection conduits 9, 13, to establish a communication from the tubes 16, 17 to the nozzle 2 through the connection block 6 and the pipe 5. A bore 8 of the connection block 6 is sized to accommodate the proximal end 7 of the nozzle pipe 5 with a relatively close fit, but preferably releasable by hand. The nozzle pipe 5 is provided with annular seals 21, 22, 23, arranged in annular recesses in the nozzle pipe on either side of the openings 18, 19 to avoid leakage of the fluids. The openings 18, 19 communicates with annular re¬ cesses, which in turn communicates with the connec- tion conduits 9, 13, irrespectively of the angular orientation of the nozzle pipe 5. In the embodiment of Fig. 3, of which an enlarged part view is seen in Fig. 7, the nozzle pipe 5 extends through the connec- tion block 6. The proximal end 7 of the nozzle pipe 5 is provided with a threading and projects from the connection block 5. The threading is engaged by a nut 15 for securing the nozzle pipe 5 in the connection block 6. The nut 15 may be provided with a hexagon opening 28 for engagement with an Allen key. Between the nut 15 and the connection block 6 a spring washer 27 is arranged whereby free mutual rotation of the connection block 6 in relation to the nozzle pipe. By free mutual rotation is meant that the connection block 6 is free to be pivoted about the longitudinal axis 34 of the nozzle pipe in any of the two direc¬ tions. It should be understood, however, that nor¬ mally there will exist a certain degree of friction between the parts, so a force of a certain degree must be used to pivot the parts. With this embodiment a relatively simple, cost-effective and intuitive se¬ curing of the nozzle pipe 5 in the connection block 6 is provided. Alternatively the proximal end 7 of the nozzle pipe may be provided with a radial hole 29 for en¬ gagement with a cotter pin 30, as illustrated in Fig. 8. This embodiment will provide a cost effective se¬ curing of the nozzle pipe in the connection block, and still allow free mutual rotation of the block 6 in relation to the nozzle pipe. Another alternative embodiment can be seen in Fig. 9. In this embodiment a circumferential groove 31 is provided at the proximal end 7 of the nozzle pipe. In this embodiment the connection block is pro¬ vided with a blind hole into which the proximal end 7 of the nozzle pipe is inserted, so contrary to the other embodiments the proximal end 7 of the nozzle pipe does not extend through the connection block to project at the back side thereof. A securing pin 33 may be inserted in an opening 32 of the connection block 6 for engagement in the circumferential groove 31 for releasable retention of the proximal end 7 of the nozzle pipe in the connection block 6. The cir¬ cumferential groove 31 will allow for free mutual ro¬ tation of the nozzle pipe and the connection block 6. In the embodiment of Fig. 10 the nozzle pipe 5 is manufactured by extrusion, whereby a unitary noz- zle pipe 5 is provided having built-in fluid conduits 11, 12, which will reduce the time consumption for assembling the burner assembly, and hence the cost thereof. Further the risk of leaks is reduced to a minimum as the number of joints is at a minimum. As shown the nozzle pipe 5 may in itself also constitute a nozzle holder, whereby the need for a separate noz¬ zle element is avoided, which further simplifies the system, reduces the risk of leaks and reduces the cost of the system. In the cross section of Fig. 11, the securing pin 33 is seen. The securing pin 33 is inserted in the opening 32 and arranged to retain the nozzle pipe 5 in the connection block 6. The securing pin 33 may be provided with a bent end, which will facilitate manipulation by the mechanic, and further the bent end may be inserted and retained in an additional bore in the connection block as illustrated, whereby the securing pin 33 is fixed in the connection block. As can be seen the extruded nozzle pipe comprises a central bore and a surrounding, sectional channel. In the shown embodiment, the surrounding, sectional channel is divided in three equal parts, which is found to provide a fair compromise between mechanical stability, ease of manufacture and flow area, al¬ though a greater or smaller number of partitions is possible. Fig. 12 is a longitudinal section of the con- nection block 6 and a part of the nozzle pipe 5 in¬ serted in the connection block 6 of the burner assem¬ bly of Fig. 10. In this embodiment, the nozzle pipe 5 is inserted in a blind hole of the connection block 6. As explained above, the nozzle pipe 5 is retained in the connection block 6 by the securing pin 33, which engages a circumferential groove 31 of the noz¬ zle pipe 5. As explained above the nozzle pipe 5 is manufactured by extrusion and comprises a central bore constituting the first nozzle fluid conduit 11. Only one of the channels making up the second nozzle fluid conduits 12 can be seen in this longitudinal section. When the nozzle of the burner assembly seen in isometric view in Fig. 4 is to be inspected, the nut 15 is unscrewed and the connection block 6 retracted from the nozzle pipe 5. Hereby the nozzle and nozzle pipe is disconnected from the supply from the source (not shown) , and the nozzle and nozzle pipe is then free and can be withdrawn from the furnace. Often the burner assembly comprises a mounting bracket 24. Preferably the mounting bracket 24 com¬ prises a threaded hole 25 for cooperation with a bolt inserted in a cover of the furnace, as it is common practice. The mounting bracket 24 may further be pro¬ vided with a scale for precise location of the nozzle 2 in the furnace. In case of an liquid fuel burner using an atom- izing gas, such as air, the liquid fuel is not pres¬ surized, and risk of spillage of liquid fuel is hence minimal, even when the connection block 6 is re¬ tracted from the nozzle pipe as outlined above. If, however, an even lesser risk of escape of fluid is desired, especially in case of a pressurized fluid, the connection block 6 may be provide with a solenoid valve 26 as seen in Fig. 5 and 6. The solenoid valve will positively cut off the supply of fluid at the connection block 6. In an embodiment of the burner assembly, a plastic tube having an internal diameter of 2 mm and an external diameter of 4 mm was used for delivery of oil. For supply of atomizing air, a plastic tube hav¬ ing an internal diameter of 3 mm and an external di- ameter of 5 mm was used. The opening 8 has an en¬ trance diameter of approximately 10 mm, whereas the opening at the backside measures approximately 8 mm in diameter, and the nozzle pipe has a corresponding step in outer diameter at the proximal end thereof. The total length of the burner assembly was approxi¬ mately 180 mm. The dimensions of the connection block were approximately 20 x 20 x 10 mm. In a test-version of the burner, approximately 20 1 of air per minute was delivered to the nozzle at a pressure above atmospheric of 0.5-0.7 bars. A stepless modulation of the burner from 600 W - 20 kW was achieved, which means that oil burners of this type are competitive with gas burners. Further it was found that requirements regarding the design and manufacture of the nozzle are less strict, and hence more feasible and economic, compared to the common pressure-atomizing burner. It is expected that a variation of output from e.g. 400 W to 30 kW with the same nozzle is possible. Further it is expected that an output of up to 1,000 kW is possible, but it is expected that this would require different nozzles for the low and high ends. The invention is not restricted by the above detailed description. For example it would be evident to the skilled person to provide a burner assembly having a further supply line, such as for dual-fuel use.