Ouranosinc / xclim / 13185465372

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Exportable function for fao_allen98 (#2067)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #2004
    - Also see discussion #1853 for more information
- [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] CHANGELOG.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?

* Adds a new function `fao_allen98` that implements the FAO-56
Penman-Monteith equation and is used inside
`potential_evapotranspiration` for the `allen98` method. This allows
providing some variables directly instead of using estimates e.g. tas
mean or the different vapour pressures can be taken from datasets
directly instead of being estimated. See discussion #1853 or issue
#2004.
* Fixes a bug where hurs provided in `%` lead to wrong results for PET
when using `allen98`

### Does this PR introduce a breaking change?
No.

### Questions:
I would like to fix the units of the input for the new function exactly.
Is there an easy way to do this? And how can I provide a scalar value as
a default with units?

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