Ouranosinc / xclim / 8930203821

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Proper use of `loc` parameter in `gamma`,`fisk` dists (SPI/SPEI) (#1720)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #xyz
- [x] Tests for the changes have been added (for bug fixes / features)
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGES.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?


* Spei used to rely on an offset to ensure that distribution of values
are positive when using distributions such as `gamma, fisk`. Now,
instead, we properly use the `loc` parameter in those distributions that
generalize the distributions and allow negative values. For instance,
the `gamma` disitribution is a 2-parameter function, but in ``scipy``
it's generalized to 3 parameters, with a `loc` parameter. This extends
the support of `gamma` from $x \in [0,\infty]$ to $x \in [\text{loc},
\infty]$.

The code was already compatible with this idea, just had to change
``_fit_start`` so it can compute estimates of the parameters in the
cases with negatives values.


### Does this PR introduce a breaking change?

* Removing the offset from SPEI. Also, the `_fit_start` can now compute
an estimate of `loc` which can affect what value we obtain when
optimizing 3-parameters distributions. Even when the `loc` is fixed to 0
with `floc=0` in `fitkwargs`, the estimate of `fisk` was modified. The
previous formulation was obtained by applying more or less the idea in
the method of moments, but now I obtained an approximation that's more
rigorous.

* The case of zero inflated distribution has been changed: Now, it is
the calib... (continued)

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