Save/load comparison objects with pickle

[alejoe91]

Closes #2012

Turns out that all comparison objects (since all sorting objects are now pickleable), are also directly pickleable. This PR only deprecates the save_to_folder/load_from_folder functions of the MultiSortingComparison.

# generate
sorting = si.generate_sorting()
sorting_mono = si.generate_sorting(durations=[10])
rec, sort = si.generate_ground_truth_recording()
we = si.extract_waveforms(rec, sort, mode="memory", progress_bar=False, return_scaled=False)

# two sorters
cmp = si.compare_two_sorters(sorting, sorting)
cmp_l = pickle.loads(pickle.dumps(cmp))

cmp_gt = si.compare_sorter_to_ground_truth(sorting, sorting)
cmp_gt_l = pickle.loads(pickle.dumps(cmp_gt))

# multiple sorters
mcmp = si.compare_multiple_sorters([sorting, sorting, sorting])
mcmp_l = pickle.loads(pickle.dumps(mcmp))

# collision and correlograms
cmp_coll = si.CollisionGTComparison(sorting_mono, sorting_mono)
cmp_coll_l = pickle.loads(pickle.dumps(cmp_coll))

cmp_corr = si.CorrelogramGTComparison(sorting_mono, sorting_mono)
cmp_corr_l = pickle.loads(pickle.dumps(cmp_corr))

# templates
cmp_t = si.compare_templates(we, we)
cmp_t_l = pickle.loads(pickle.dumps(cmp_t))

mcmp_t = si.compare_multiple_templates([we, we, we])
mcmp_t_l = pickle.loads(pickle.dumps(mcmp_t))