Thresholded events - Quantified rate2amount

[coxipi]

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)
• CHANGES.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?

• New thresholded_events generic compute and find_events in run_length. The idea is to find all runs defined by a start and a stop condition and return them along an "event" dimension.
• It relies on a new function in run_length that can compute the run_length of sequence determined by two conditions. A first condition determines when runs should start, and a second one determines when they stop. This is called runs_with_holes as one possibility with the addtition of the second condition is this: consider a normal run_length, but the second condition allows for holes in thoses sequences, where those holes cannot exceed a given threshold window_stop. runs_with_holes is in fact more general, so perhaps another name should be found.
• rate2amount and amount2rate can now accept Quantified (str and Quantities), which then requires passing the time coordinate in as the dim argument.

Does this PR introduce a breaking change?

No

Other information:

For the time being, I removed the references to "freezing rain" as I'm not sure I will need the function in xclim. If I do, then, I'll add it back.

[aulemahal]

@coxipi So I added the ability to find events for each resampling period. I merged in resample-map thinking I could use that stuff here, but as the new functions are multivariate and output a Dataset, it doesn't work...

I edited to the comment with some more info.

[aulemahal]

After discussion with Mrs Verglas herself, I decided that implementing a freezing rain indicator here was not worth the hassle:

• No standard variable name for freezing rain
• Very specific and niche
• The best xclim implementation might not be the optimal performant one, considering that we don't need all outputs of the find_events
• The potential need to perserve the events would bypass an indicator implemented here anyway

But I did add an application for runs_with_holes by adding yet another parameter to the spell length statistics.

[coxipi]

@aulemahal approved (can't approve this PR, I started it)

[Zeitsperre]

Some minor modifs, but looks good from my perspective