Usecase/optical throughput calibration

This merge request is partially closing issue 'Atmoshpere and throughput calibration'. I decided to split this issue in 2 parts, one is optical throughput calibration that is addressed by this request, and the other is atmosphere calibration, which will be addressed by a future request. This request consists of 2 use cases, the primary one using muon rings, and a complementary one using the Cherenkov Transparency Coefficient. Both can be used in parallel, since they do not interfere with normal data taking. In the future it can include more use cases (e.g. intercalibration and / or crosscalibration using eventwise comparisons of shower image amplitudes) if CTA decides to apply them.