Switched to stocastic energy smearings.

Also a couple of fixes/small improvements:
- MET tag was using uncorrected MET when evaluating *any* systematic.
  Switched to always use corrected MET. Need a dedicated smearer (or simple flag) for MET systematics.
- Protect against missing histograms in SimultaneousSignalFitting/test/calcPhotonSystConsts.cpp
- Automatic booking of Full MVA tree variables (in synch with used smearers).
Merge pull request #267 from musella/topic_switch_stocastic_smearing

Switched to stocastic energy smearings.