Resolve "Making impacts working again"

Release notes

• Fixed impact calculation loading nominal snapshots when it shouldn't creating essentially all-zero impact values
• Changed impact calculation to use proper, minos based parameter errors for determining +/-1 sigma values of parameters of which impacts should be measured.

Details

• Some refactoring of how the snapshots are created in the runPreFit method of TSImpactCalculator
• Changed (default) snapshot names used therein to include "Impact" in their name to avoid accidentail re-use of unsuitable snapshots from TSBreakdownCalculator (see diff to get an idea of the subtleties involved).
• No longer run a dedicated fit to create the conditional snapshot, but only change const pars settings w.r.t. the unconditional one. This is to not overwrite the errors determined (via minos) in the former fit -> in a (almost-)all-const fit we wouldn't get meaningful errors of the NPs anyways. Thus this saves time and makes the behavior more understandable and predictable (e.g. no longer depend on how RooFit (does not) change errors of const pars during a minimization).

Closes #342 (closed)