Added support for fixed-target mode using Unigen reader

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In simulation/generators/mpdGen/MpdUnigenGenerator.*:
- Added flag isLabSystem in constructor for fixed-target mode;
- Added Lorentz boost (cm -\u003e lab) for fixed-target mode if the flag isLabSystem is set.
In macros/common/runMC.C:
- Added isLabSystem in the unigen reader.