Update customization for GEM+ME0 digitizer

Updated customization file to run: 
* only GEM or GEM+ME0 digitizer
* GEM or GEM+ME0 digitizer on top of Muon digi sequence
* GEM or GEM+ME0 digitizer on top of Full digi sequence

This file is only referenced in SimMuon/GEMDigitizer/test/runGEMDigiProducer_cfg.py.