get_warming_level

[RondeauG]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Make it possible to search within IPCC_annual_global_tas.csv without data #208
• (If applicable) Documentation has been added / updated (for bug fixes / features)
• This PR does not seem to break the templates.
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• subset_warming_level got separated in 2: get_warming_level to search in the CSV and subset_warming_level for the subsetting itself. The main difference is that get_warming_level accepts strings or lists of strings, on top of xr.Dataset.

Does this PR introduce a breaking change?

• Only if someone called subset_warming_level with purely positional arguments. As to not repeat arguments twice, most of them (window, tas_baseline_period, etc.) are now in get_warming_level, while subset_warming_level makes use of kwargs.

Other information: