Python3 + flake8 ElectronPhotonFourMomentumCorrection

Ruggero Turrarequested to merge
turra/athena:fix_ElectronPhotonFourMomentumCorrection_python3_flake8 into master

Several scripts used to test ElectronPhotonFourMomentumCorrection tool have been properly ported to python3 and flake8 warnings have been fixed. Now the scripts use the latest matplotlib api presently provided by cvmfs.

Closes ATLASG-1579

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