Patch for DD4Hep #862: do not change momentum in getParticleDirection

Isotropic DDG4 gun generator cannot be run with fixed energy. We rely on that for single particle production.

Upstream issue: https://github.com/AIDASoft/DD4hep/issues/862

No PR: not sure how they'll want to handle this upstream.

assigned to @sly2j

enabled an automatic merge when the pipeline for 35eb953f succeeds

merged