Bug fix for GED electron MVA preselection calculator

Properly initialize and use some variables that were missed in the calculation of the electron preselection-mva.

These variables were present in the training, but not in the evaluation, so they carry some rejection power.

Small expected changes for tracker-driven only electrons, small improvements in efficiency/fake-rate.

Now includes #2717 and #2719 .

We should backport this to 700.