ETCCDI indices

[mnurisso]

PR description:

Implementation of CLI scripts to evaluate ETCCDI indices as described here.
Only a preliminar list of indices is available.

They work at the moment with native healpix data and no regrid, but this can be easily extended.

A more rigorous structure as a class, with definition yaml files and only a final cli to run as a proper diagnostic would be beneficial.

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• environment.yml and pyproject.toml are updated if needed, together with the lumi installation tool.
• Diagnostic config files path are updated in the console if needed

[jhardenberg]

@mnurisso sorry if I see and comment on this one only now. The all seem very simple but computing these indices can be very tricky .... there is actually a nice library which does this and which we could simply call: https://xclim.readthedocs.io/en/stable/

[jhardenberg]

Also the old ETCCDI set of definitions was later superseded by ET-SCI indices (a superset actually).

[jhardenberg]

The main problem in computing them is those which depend on percentiles and fine details on how percentile boundaries are computed.