Herwig7_i: add custom pwg_reader and pwg_reader_lib option in _lhef_commands

In order to use the existing lhef_powhegbox_commands in bb4l+H7 MC job options see also !72582, which require a custom LHEReader and custom shared library, two new options are introduced pwg_reader and pwg_reader_lib to _lhef_commands and lhef_powhegbox_commands. This should not influence existing job options as the defaults are set to the previously used values.