[SCM] add 3 consecutive nb_days as default argument. add this argument for SWE and SD. Also add a test to SCM study

[SCM][HYPERCUBE] Add multiprocessing by default for abstract study. re-activate test_SCM_study which are now fast enough. fix issue with total precipitation variable. AltitudeHypercubeVisualizer first visualization.

[SCM] add permutation which correspond to the sum of rainfall & snowfall. add test based on Durand paper.