premix relvals: adapt to fixes of premixing

. process string set to differentiate 25ns/50ns pilep overlaid when done with premixed events (affects only wf injection to reqmgr)
. --geometry Extended2015 as needs be in the 70x (all GT-driven in subsequent releases)
. introduced SLHCUpgradeSimulations/Configuration/postLS1CustomsPreMixing wherever needed by premix workflows to adapt to Mike's #3973; ** TEMPORARY **