ENH: Add units_metadata attribute to the output of temperature based indices

[bzah]

• icclim version: 6
• Python version: n/a

Description

CF has added a units_metadata attribute that is used to differentiate a difference of temperatures and a on scale temperature.
See: units_metadata cf-convention/vocabularies#172

The temperature: difference concerns mainly DTR, vDTR, ETR but also some generic indices.
The other indices should still set this attribute but should set temperature: on-scale if they are computed on temperatures.

As for the input of icclim, either we assume it's always temperature: on-sclae daily data (or whatever frequency) or we add a check for this units_metadata but it won't be filled for existing datasets.