feat(grib): Improve template manager

[gmertes]

Description

Improve the grib template manager in a backwards compatible way.

• Improve documentation on how users can provide their own templates
• For the samples provider, add the option for the sample index to be specified directly in the inference config
• For the file provider, add an option for the file to contain many parameters, and do a lookup based on the param name.
• In the case of grib input, the template manager automatically injects templates found in the input grib at the highest priority. Refactor this so that this becomes its own standalone template provider called input, this gives more flexibility to enable/disable combinations of template providers and set the priority..
• input provider fallback mode: force an output variable to use one of the input variables
• Improve logging of the template manager. Right now it only logs in the case of errors. Also log in the case of success so it's easier to see which templates are being used from which provider.
• tests for everything, including backwards compatibility

As a contributor to the Anemoi framework, please ensure that your changes include unit tests, updates to any affected dependencies and documentation, and have been tested in a parallel setting (i.e., with multiple GPUs). As a reviewer, you are also responsible for verifying these aspects and requesting changes if they are not adequately addressed. For guidelines about those please refer to https://anemoi.readthedocs.io/en/latest/

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📚 Documentation preview 📚: https://anemoi-inference--383.org.readthedocs.build/en/383/