Hgc digi devel

This pull contains a first implementation of the pulse shape + some fixes necessary to properly take into account the time assignment given by the MixingModule and to properly compute the time of flight.
By default the configuration is disabling the pulse shape as we're testing different alternatives 
and checking the one which should properly be used for the performance studies.
The pull is being made as we would like to freeze a working setup for official samples request
while we sort some of the details in parallel.
It was developed on top of CMSSW_6_2_0_SLHC16 and privately tested with 
runTheMatrix.py -w upgrade -j 3 -l 12200,12210
and also with a local setup where an average of 140 MinBias events were mixed.

@vandreev11 @ramankhurana @bsunanda follow this as well.