Consider allowing specifying 'tagname' from `GRID3D_AVERAGE_MAP` config

> The ‘name’ in the setup must be unique, as it is a dictionary. This name becomes the data-tagname in the metadata. See example below:
> I don’t know if it matters, but usually the tagname is something like “ecl_average_SWAT”, i.e., without the date. Thus, in the example: “pem_average_TWTPPDIFF”.
> My workaround here is another custom ERT forward model that fixes the tagnames in the metadata and also renames the files accordingly.
>
> Suggestion:
> Allow for a manual metadata tagname in the config file for the GRID3D_AVERAGE_MAP forward model, which overwrites the default name and is also used to generate the filenames.