Doc/document generic indices

[bzah]

Pull Request to resolve #206, #203

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

Describe the changes you made

• Added generic indicators as part of icclim namespace
• Added per function documentation for generic indicator
• Auto-generate both features above with the usual extract-icclim-fun.py script
• Clean-up some documentation
• Added some user friendly sphinx modules (shamelessly copied from xclim)

[bzah]

Failure of CI due to https://github.com/Ouranosinc/xclim/issues/1211 . We are going to pin pint under 0.20 until xclim 0.39 is released.

[github-actions[bot]]

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	29	1	97%	20
icclim/generic_indices
generic_indicators.py	470	84	82%	57, 60, 77, 81, 85, 98, 104, 178, 219, 226, 357, 386, 668, 699, 705, 713, 732, 895, 915–921, 925, 955, 957, 1022–1023, 1056–1083, 1110–1131, 1147, 1156–1157, 1163–1193, 1199–1203, 1227
standard_variable.py	46	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	96	10	90%	102, 146, 157, 191, 197, 235, 239, 267–268, 305
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	2456	219	91%

Test results

Tests	Skipped	Failures	Errors	Time
185	0 :zzz:	0 :x:	0 :fire:	2m 10s :stopwatch: