Standardize state diff stats in mdp datastore. Change variable name.

[SimonKamuk]

Describe your changes

This PR fixes a bug where the MDPDatastore returns non-normalized state_diff_mean and std, while NpyFilesDatastoreMEPS returns the normalized versions. See the linked issue for a discussion on this.

The fix updates the MDPDatastore to return state_diff_std_standardized and state_diff_mean_standardized in addition to the existing non-standardized versions. The NpyFilesDatastoreMEPS already only returned the standardized versions, so here the variable name has just been changed.

Also updated where get_standardization_dataarray is called to get the standardized versions.

Issue Link

Closes #112

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[SimonKamuk]

I think this is ready for review. One question I have is whether I should keep both the normalized and non-normalized state_diff stats (this is what I've done), or if I should just remove them entirely.

[SimonKamuk]

Also, this goes in v0.4.0, right?

[joeloskarsson]

See small comments above (forgot to make them part of review)

[SimonKamuk]

Done 👍 care to have a final look and then I'll merge?

[joeloskarsson]

Perfect! lgtm