[COMPARISON] add class_property to abstract study, to enable spatial display of errors between SAFRAN reanalysis & data from observations

[SCM][HYPERCUBE] Add multiprocessing when loading abstract study. Refactor abstract variable and all variables accordinlgly.

[SCM][HYPERCUBE] Create Hypercube object with MultiIndex. Modify @cached_property in utils (from a library and seems to a bit faster). added some parallelization for trend test type in study_visualizer.

[DATA] add safran extended (which include as massifs: the original massif + the 4 alps regions + the whole alps)