egammaRec+egammaTools: cmake fixes and enable flake8

Frank Winklmeierrequested to merge
fwinkl/athena:fwinkl_20201126T115501 into master
  • egammaTools: Remove CLHEP dependency by using Gaudi's SystemOfUnits.h. Update cmake configuration and enable flake8.
  • egammaRec: enable flake8

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