[ENH] Bind xclim indicators to missing ECAD indices

[bzah]

Draft how we should bind existing xclim indices into icclim v6 to complete the ECAD list.

Pull Request to resolve #203

• Unit tests cover the changes.
• These changes were tested on real data.
• The relevant documentation has been added or updated.
• A short description of the changes has been added to doc/source/references/release_notes.rst.

Describe the changes you made

• Add some ECAD indices that can be easily bind to xclim's indicators.
  This includes {GSL, SPI3, SPI6}.
  The same approach could be used for other missing indices that can't be generified and that are available within xclim.
• Add some documentation for generic indices.

This PR is also related to #204 but does not fully resolve it.

[github-actions]

Report

File	Stmts	Miss	Cover	Missing
icclim
icclim_exceptions.py	7	1	86%	20
icclim_logger.py	85	29	66%	24, 43, 65, 70–95, 105, 111–142, 150–155
main.py	270	20	93%	119, 128, 404, 489, 515–516, 518–519, 521, 523–524, 526–527, 546–553, 690, 697, 701
utils.py	37	5	86%	22, 33–34, 36–37
icclim/ecad
xclim_binding.py	48	6	88%	39, 43, 68, 72, 97, 101
icclim/generic_indices
generic_indicators.py	472	85	82%	57, 60, 77, 81, 85, 98, 104, 178, 219, 226, 350, 379, 669, 700, 706, 714, 733, 896, 901, 921–927, 931, 961, 963, 1028–1029, 1062–1089, 1116–1137, 1153, 1162–1163, 1169–1199, 1205–1209, 1233
standard_variable.py	48	1	98%	19
threshold.py	398	21	95%	198, 218, 222, 270, 286–289, 304, 311, 387, 526, 595, 691, 894, 916, 922, 996, 999, 1097–1099
icclim/models
cf_calendar.py	35	1	97%	20
climate_variable.py	100	10	90%	102, 161, 172, 206, 212, 250, 254, 282–283, 320
frequency.py	176	11	94%	167–170, 314–316, 323, 401, 427, 432
index_group.py	34	1	97%	33
operator.py	29	1	97%	14
standard_index.py	28	3	89%	56, 59, 63
icclim/pre_processing
input_parsing.py	178	33	81%	41, 105, 110, 116, 125, 138, 167, 183, 197, 217, 225, 227, 236, 245, 263, 277–285, 294–309, 331
rechunk.py	86	2	98%	38, 132
TOTAL	2530	230	91%

Test results

Tests	Skipped	Failures	Errors	Time
186	0 💤	0 ❌	0 🔥	2m 8s ⏱️

[bzah]

@pagecp I've added in the documentation a "how to" recipe for generic indices. It's not fancy but at least it gives an overview of each generic index.

See https://icclim--235.org.readthedocs.build/en/235/how_to/recipes_generic.html

[pagecp]

It is really great! It will really help to startup a more full documentation later. And at least current users will be able to use those as examples.