Fix/unit for diff based indices

[bzah]

Pull Request to resolve #255

• [x] Unit tests cover the changes.
• [ ] These changes were tested on real data.
• [ ] 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

Indicators computing the difference between two variables must first convert the units of input variables to the expected output unit in order to avoid converting the output which would be a relative "temperature". In other words: a 15 Kelvin difference is equivalent to a 15 degC and is not to a -258.15 degC.

[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	277	26	91%	119, 128, 404, 489, 515–516, 518–519, 521, 523–524, 526–527, 546–553, 611–616, 697, 704, 708
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	482	85	82%	57, 60, 77, 81, 85, 98, 104, 178, 218, 225, 359, 388, 678, 709, 715, 723, 743, 906, 911, 931–937, 941, 971, 973, 1038–1039, 1072–1099, 1126–1147, 1163, 1172–1173, 1199–1229, 1235–1239, 1263
standard_variable.py	48	1	98%	19
threshold.py	398	21	95%	199, 219, 223, 271, 287–290, 305, 312, 388, 529, 598, 694, 897, 919, 925, 999, 1002, 1101–1103
icclim/models
cf_calendar.py	35	1	97%	20
climate_variable.py	100	10	90%	103, 162, 173, 207, 213, 251, 255, 283–284, 321
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	195	37	81%	74, 78–79, 84, 103, 167, 172, 178, 187, 200, 229, 245, 259, 279, 287, 289, 298, 307, 325, 339–347, 356–373, 395
rechunk.py	86	2	98%	38, 132
TOTAL	2564	240	91%

Test results

Tests	Skipped	Failures	Errors	Time
187	0 :zzz:	0 :x:	0 :fire:	2m 0s :stopwatch: