Update RecoTracker/dEdx package

Update RecoTracker/dEdx package in order to reduce the memory and CPU footprint,
Following suggestions by Vincenzo Innocente.
Also cleaned up the python and test directory.
Changes were tested in CMSSW_7_2_X_2014-08-06-0200 on standard RECO sequence.

Please include these changes in future 720_preX releases.
Thanks,
check to not divide by zero