ASMODEE: enable tuning of calibration window

[thibautjombart]

Ideally, the calibration window should include a single trend which can be modelled by at least one of the candidate models. In practice, this may require selecting a subset of the most recent data, e.g. 6 weeks prior to the prediction window. We might add an optimisation procedure to get the best suited calibration window, e.g. based on absolute fit.