Significant speed-up to switch systematics in BTagging tools

Two changes that can produce significant speedup in xAOD processing codes:

• applySystematicVariation was quite slow as it always treated the complicated cases, while it can be much faster for the simple and (common) case where the user is passing the right thing.
• loop on systematics in truth tagging tool was super inefficient as it called applySystematicVariation twice per jet, always putting syst back to nominal. Now it's the responsibility of the systematics loop to do so.

These changes should be ported to r22, I presume.