perf(ekd): Select datetime before cloning fields

[gmertes]

Description

A small performance optimisation: when selecting variables in the ekd input, only clone the dates that we need instead of the whole fieldlist.

Impact is minimal in a forecast configuration, where the input fieldlist only contains a few steps.

But for the temporal downscaler, where the input fields can be a large forecast, this avoids unnecessary cloning of the whole fieldlist, resulting in a small speedup.

Also clean up some of the typings, and remove the old FieldArray in favour of the new FieldList and SimpleFieldList (to be further cleaned up in the 1.0 migration, but this is a first pass).

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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[gmertes]

You know the PR is fire when the test segfaults 🔥

[HCookie]

Can this be reviewed, it seems the failure is isolated to the cosmo tests?