Add feature to evaluate network for specfic, named snapshots.

[Tetra-quark]

Adds a feature to pass a list of snapshot names snapshots_to_evaluate to the evaluate_network function as an alternative to using the snapshotindex parameter in the main config.yaml file.

The new parameter allows multiple, explicitly named snapshots to be evaluated where previously only a single snapshot or "all" could be chosen.

This pull request closes #2356.

Overview of changes:

• Add snapshots_to_evaluate parameter to both single and multi-animal evaluation.
• Function get_available_requested_snapshots to handle the snapshots_to_evaluate parameter.
• Function get_snapshots_by_index to refactor existing snapshotindex from config file functionality.
• Function get_snapshots_from_folder to refactor getting an ordered list of snapshots from the train folder.
• Docstring for new functions and parameters.
• Unit tests for all three new functions.
• Update testscript.py to use snapshots_to_evaluate in evaluate_network (however in doing so I have disabled the snapshotindex method, both should be in the test ideally).
• General refactoring, renaming and cleaning.

Example usage:

deeplabcut.evaluate_network(
    config_path, 
    snapshots_to_evaluate=["snapshot-50000", "snapshot-100000"],
)

[MMathisLab]

@n-poulsen and @jeylau let's prioritize this -- I think it's a great addition, and can be worked into DLC3.0