Add option to use CMSSW based combine

This PR adds a feature to the setup to opt for a CMSSW based combine installation. When doing the default setup (just source setup.sh without any setup "name"), the standalone version is picked.

By mistake, the PR also contains a feature to the pulls & impacts task that allows for using the initial fit as a snapshot for all following nuisance fits, as well as a few minor improvements.

added feature label

assigned to @mrieger

mentioned in commit dbced1b5

merged