Ouranosinc / xclim / 11804157644

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Fix wrong unpickling with dask 2024.11 (#1993)

### 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.

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