Add capability to shoot beam electrons starting upstream of magnet, with settable energies

[bloodyyugo]

For the off-energy beam-electrons study we need to start the electrons before they feel any field from the analyzing magnet.

We need to add a single-particle gun configuration that starts upstream of magnet with correct position & direction for nominal (8GeV , arriving at center of target) electrons that also has settable electron energy.

This can all be done in generators.py and we already know the position and direction at ~8m upstream of target (plenty of distance for zero field effect). It's just a few lines of python given the existing functions.