Merged requested to merge mmuskinj/athena:21.0-StepHistograms-PreStepPoint into 21.0
As discussed with Geant4 authors, @jchapman, and @hgray, it makes more sense to use the "PreStepPoint" instead of "PostStepPoint" for the following variables:
- kinetic energy in current step,
- volume of the current step,
- step position.
For the process corresponding to the current step "PostStepPoint" remains to be used. I also added kinetic energy at the end of the step (using the "PostStepPoint") because it adds some interesting info. This is how it was calculated before, but I did not want to completely remove it, so now we have both pre- and post-step kinetic energy.