Add a --which_savepoint option to allow narrowing test cases on many-savepoints dataset

[FlorianDeconinck]

We can record many timesteps (call savepoints in code) in one dataset for a specific translate case. While useful for things moving slowly like physics it can lead to overly long tests. We shall implement a which_savepoint option, mimicking existing which options that narrow the tests.

Parent: GEOS-ESM/SMT-Nebulae#41

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• --which_savepoint allows to run one or many (string delimitated) savepoints