Ouranosinc / xclim / 5094954857

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Accelerate import - prevent numba warnings (#1378)

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### Pull Request Checklist:
- [ ] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes an issue raised verbally by `xscen` developers.
- [ ] 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?

PR #1135 removed all signatures where possible, so that `@jit`-decorated
function would be compiled at run time (you know "just in time"...).
However, `@guvectorize` -decorated functions _must_ have signatures and
thus must be compiled at compile time, which means upon import.

This PR adds `cache=True` to these function, so numba compiles them once
and than saves the compilation to a `__pycache__` folder if possible.
The first import will be slow as before, but subsequent imports should
be much faster.
I'm sure there are edgecases where the user of xclim doesn't have write
access to the source folder, which might mean this caching will be
unavailable. But for most users, this should be a nice performance
boost.

On my machine I went from 13s to 2.5s.

This PR also forces `nopython=True` to these `@guvectorize` functions.
Numba 0.59 will change the default value of this parameter and it was
raising warning complaining about our lack of precision.

### Does this PR introduce a breaking change?
It shouldn't. But I do not know all the ramifications of caching these
functions...

### Other information:
@RondeauG, fyi.

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