[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 snowfall] add main_sensitivity_one_altitude_range.py. add detail plot for each massif for the return level

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

[projection snowfall] add preliminary analysis for the work on model with effect related to the GCM or RCM type.

[projection snowfall] improve one_fold_fit_merge.py. improve interval for the sensitivity plot. Add is_temperature_interval and is_shift_interval to define the intervals.

[refactor] rename folders for one/two fold fit. move the code for the anomaly into temperature_covariate.py

[refactor] move altitudes_studies.py. create trend test folder inside extreme_trend. deactivate one test for param_function that was crashing globally for unknown reason (but locally the test is working).