Usecase/end to end calibration

First draft for end to end calibration usecases. There are 2 usecases grouped together, one addressing the site cross calibration, and the second the optical throughput calibration using electrons. They might also be grouped together with the other optical throughput calibration usecases, we can think about it. Usecase for cross calibration with different observatories / experiments is not included yet, since it is not clear we are going to implement it.

Closes #43