MpdUnigenGenerator: added support for fixed-target mode (Lorentz boost)

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MpdUnigenGenerator: Added support for the fixed-target mode.
- Input model data (UniGen) is the same for both collider and fixed-target modes (i.e. in ecm system)
- Added new flag isLabSystem in the constructor of MpdUnigenGenerator
- If isLabSystem is true the Lorentz boost (ecm-\u003elab) is performed while reading input model data