Add GRIB templates for ICON-CH1

[frazane]

This pull request adds a new script to help generate GRIB templates for the ICON-CH1 model data, along with a set of templates ready for use. It also adds a templates index for ICON-CH1 data to be used by anemoi-inference.

[frazane]

@dnerini this is ready, however please note the following.

The GRIB templates are currently managed in anemoi-inference via the "template indexes" YAML files. Up to now, we have defined separate indexes for COSMO and IFS templates, however I am not sure this is a good path going forward. It would be ideal to have all templates under the same index and have a robust mechanism for anemoi-inference to automatically determine which ones to use. In theory it's there, in practice I haven't been able to use it to distinguish between e.g. COSMO and IFS or COSMO-1 and COSMO-2 grids.

For now, I included a separate index for the ICON templates, so we can quickly test that we can write the output files correctly. I suggest we address this problem in a separate PR, and importantly we should probably coordinate with Gert who is reworking the template management upstream: ecmwf/anemoi-inference#383