Check that required fields are present in IMAS geometry loader

We should check required fields are present and raise an error if they are not.
Currently, if e.g. gm7 is not present in the equilibrium IDS, the config builder will fail with a shape error that is hard to pin down.

Currently checking of many fields is done lazily, but many fields have also been missed. I wonder whether it would be better to do eager checking at the start?

Start at the IMAS geometry loader and inspect how required fields are currently checked lazily. Reproduce the missing-gm7 case in the equilibrium IDS, then identify the required fields and ensure absent fields produce a clear error before the config builder reaches a shape error.

Written by the indexing model from the issue text.