Fix lagging during metrics computation

[RagnhildHolden]

• Added a config parameter results_save_frequency to select whether results should be stored for every patient or epoch, to avoid file writing and dataframe concats for each patient and speed up processing in large datasets.
• Added lookup dicts for checking which patients were already computed to reduce dataframe scanning.
• Option to init PatientMetrics from a passed dataframe to avoid reading the file once for each patient for each resume check + is_complete() check moved before __identify_patient_files(), this speeds up the checking of already computed patients / cases.

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