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Smooth transition between ice and water formulas for saturation vapour pressure + Dewpoint indicator (#2206)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #2165
- [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?

* New arguments `interp_power` and `water_thresh` for
`saturation_vapor_pressure` and all related functions. When given,
`interp_power` controls the shape of the transition between ice and
water as reference for the saturation vapour pressure formulas.
+ Deprecated 4 letters methods, but only from the doc. A conversion is
done to support them anyway.
* Rewrite of the e_sat function to make this simpler (?) and extensible
* New methods "buck81" and "aerk96".
* New method "ECMWF" that combines both.
* New indicators:
   + `vapor_pressure` : To compute the partial pressure of water vapor.
+ `dewpoint_from_specific_humidity` : To get `tdps` from `huss` and
`ps`.


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

I changed the value for $\epsilon$ in `specific_humidity` and
`specific_humidity_from_dewpoint` so all three uses of the same constant
are now identical. The change is on the 5th decimal, I don't think it
really "breaks".

I also changed the "var_name" of indicator
`specific_humidity_from_dewpoint` to `huss`, which makes more sense!

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