13 TeV relvals: recycle for guns and new mag field

These changes are not urgent nor are bug fixed; yet please have them included in whichever forthcoming release along 620 - so we keep the release consistent with what we're using for relval production.
Changes:
- recycle for particle guns samples
- treat halo and minbias @13 TeV consistently
- introduce 38T_PostLS1 in step1; leave it commented out in subsequent steps => needs GEN-SIM from 620_patch1 to be consistently activated