Ouranosinc / xclim / 6617734176

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Commit Message

Improve SPI performance (#1311)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #1270 and fixes #1416 and fixes #1474
- [x] Tests for the changes have been added (for bug fixes / features)
- [x] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] HISTORY.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

* Make SPI/SPEI faster
* fit params are now modular, can be computed before computing SPI/SPEI.
This allows more options to segment computations and allow to obtain the
fitting params if troubleshooting is needed.
* time indexing now possible
* `dist_method` now avoids `vectorize=True` in its `xr.apply_ufunc`.
This is the main improvement in SPI/SPEI.
* Better document the limits of usage of standardized indices. Now
standardized indices are capped at extreme values ±8.21. The upper bound
is a limit resulting of the use of float64.

### Does this PR introduce a breaking change?
Yes.

* `pr_cal` or `wb_cal` will not be input options in the future: 

> Inputing `pr_cal` will be deprecated in xclim==0.46.0. If `pr_cal` is
a subset of `pr`, then instead of:

`standardized_precipitation_index(pr=pr,pr_cal=pr.sel(time=slice(t0,t1)),...)`,
one can call:
`standardized_precipitation_index(pr=pr,cal_range=(t0,t1),...)`.
If for some reason `pr_cal` is not a subset of `pr`, then the following
approach will still be possible:
`params = standardized_index_fit_params(da=pr_cal, freq=freq,
window=window, dist=dist, method=method)`.
`spi = standardized_precipitation_index(pr=pr, params=params)`.
This approach can be used in both sce... (continued)

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