Fix wrong unpickling with dask 2024.11

Pull Request Checklist:

[x] This PR addresses an already opened issue (for bug fixes / features)
- This PR fixes #1992
[ ] Tests for the changes have been added (for bug fixes / features)
- [ ] (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?

The issue introduced with dask 2024.11 concerned the passing of arguments through the dask graph. Most SDBA classes are children of Parametrizable, itself a child of dict. When stored in the graph, objects like this are pickled. and then repickled when the function is actually executed.

For some reason, the repickling stopped working and objects where reinstated as dictionaries instead of their subclasses. This particularly touched the Grouper, one of the only such objects actually been passed through dask-backed computations.

Inheriting from dict was once not recommended but I think this is not the case anymore. Anyway, I found that inheriting from collections.UserDict actually solved the issue (after adapting the internals). Not sure if I should raise the issue back to dask...

Does this PR introduce a breaking change?

I changed how all classes of xclim.sdba are made, but in theory the change is at a low, private level.

Ouranosinc / xclim