Add a regime calculation function

(transfer from ticket https://forge.irstea.fr/issues/5445)

It could be nice to propose a function to calculate regime (for P, T, PE, Q, SnowPack, SCA) from an OutputsModel object and Qobs (or SWE obs or SCA obs) and also to plot them. This function could have as arguments the aggregation time step and a sliding window length.