Block-mapped resample with the help of flox

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.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?

Implements resample_map. This function is meant for all da.resample(...).map(...) calls. These, flox cannot improve automatically so we use some flox logic to help. The idea is to map the resample-map construct on each block in parallel. This is possible by first rechunking the array so that chunks boundary fit with resampling period boundaries (this is a flox function).

The main improvement should come from the fact that map_blocks hides much of the complexity to dask, so the resulting graph is much lighter. I still have to better test the performance of this. My goal would be to have some short text in xclim's doc that highlights when the option is useful and when it is not. The option is activated through set_options.

The current function works only when the input object is of the same type as the output one. So some functions couldn't be wrapped with this yet. The most important untouched code for the moment is the missing checks where I think this could help a lot.

Does this PR introduce a breaking change?

It should not. This is completely optional.

Other information:

In progress, I still need to prove the performance boost.

This depends on #1845 because I need all improvements for PC.

[coveralls]

coverage: 89.472% (+0.03%) from 89.445%
when pulling 8096991 on resample-map
into dd37e5a on main.

[coxipi]

approved!