feat: abstracting accumulation

[b8raoult]

Description

This PR revisits the accumulation source to extend it to work with other backends apart from MARS. As part of this work the main outcomes and changes are:

• Interface change: Accumulation source is now named 'accumulate' and can work with both MARS and GRIB files. This new source takes 4 inputs: source, availability, patch and period.
• PR includes docs with more details on the above docs/building/sources/accumulate.rst and a recipe example docs/howtos/create/05-create-accumulations.rst

Future work will be done to extend this source to work with other backends. The new design will make this much easier as new backends will be easier to simply plug to the source.

---

📚 Documentation preview 📚: https://anemoi-datasets--326.org.readthedocs.build/en/326/

[floriankrb]

This PR is finished from my point of view and could be merged.

Many additional tests have been added and all relevant tests are passing. The API changes slightly and is documented. Another PR will be required to handle other sources and/or compute running averages, as this may lead to changing the argument passed to the sources and needs to be coordinated with the current work on observations.

@flyIchtus ?

[MeraX]

Please update the description before merging.

I mean description of this PR to get a helpful commit message.

[anaprietonem]

Please update the description before merging.

I mean description of this PR to get a helpful commit message.

Hey @MeraX description is now updated!