Add electronDecoratorAlg

The idea is to decorate electrons with information used in electron identification (arXiv:1902.04655) for future use in flavor tagging with GN1 or DL1 modifications. It was done directly in TDD, and we want to move these decorations to Athena (atlas-flavor-tagging-tools/training-dataset-dumper!425 (comment 6249178)).

It adds 6 decorators:

• et - Transverse energy of the electron
• deltaPOverP - Momentum lost by the electron track between the perigee and the last measurement point divided by the momentum at the perigee
• ptVarCone30OverPt - Ratio of isolated pt to pt
• energyOverP - Ratio of cluster energy to track momentum
• z0AlongBeamspot - Electron z0 along beamspot
• z0AlongBeamspotSignificance - Electron z0 significance along beamspot

This merge request follows the intention of moving decorations from FTAG Training Dataset Dumper (TDD) to Athena to speed up jobs (see !51948 (merged), !51627 (merged), Jira: AFT-596).

@dguest @svanstro