Herwig7_i compiling PowhegLesHouchesFileReader library with the build

(cc @cgutscho , @dhirsch , @ewelina )

Add source code and header files of PowhegLesHouchesFileReader to allow showering of multi-weight events from Powheg in Herwig7_i, instead of shipping an external .so library with the JOs (https://its.cern.ch/jira/browse/AGENE-1877). A powhegHerwig shared library is compiled in the Herwig7_i build and can be called through the usepwhglhereader argument in Herwig7Config.lhef_powhegbox_commands.

By running tests on MC JobOptions with ID 600306, it is confirmed that the Athena-based library does not modify the nominals (i.e. in comparison with the externally compiled powhegHerwig.so library or in comparison when the showering of multi-weights is switched off entirely).