Merge Knut's work on SR1 into master

[JelleAalbers]

This merges the work of @kdund's development branch for XENON1T SR1 into master:

• Ability to set an 'efficiency' shape parameter, which reduces the expected event rate for all sources which have apply_efficiency = True in their configuration.
• split_results function for LogLikelihoodSum, which takes a parameter list (e.g. output of a best fit) and splits it into parameter dictionaries appropriate for the individual likelihoods
• Ability to set different livetime_days in LogLikelihoodSum, probably for the purpose of run combinations.

I fixed a small bug in the last feature (the changed livetimes were not actually propagated)

[kdund]

Thanks!