[projection snowfall] modify altitude_class to altitude_group in visualizer_for_projection_ensemble.py. fix index for ensemble dataset.  add plot_relative_change_in_return_level.py

[projection snowfall] add functions_utils.py. modify one_fold_fit.py so that it may support one altitude fit.

[projection snowfall] adapt the one_fold_fit.py code to handle fit with a single altitude. Modify altitude_class by altitude_group, adapt tests. Modify the DefaultAltitudeGroup. adapt main_projections_ensemble.py

[projection snowfall] refactor first_year and last_year in OneFoldFit. and apply the code to check the difference in spatial patterns between safran 2020 reanalysis and ADAMont v2

[projection swe] modify interval (and data constraint) for temperature sensitivity. add invidual plots in sensitivity. add changes plot in sensitivity.