Support multiple patient identifier mappings in transmart-copy

[gijskant]

Currently, transmart-copy fails if multiple identifier mappings are used for patients. In that case there will be multiple rows in patient_mapping.tsv for the same patient, which is rejected by transmart-copy: it expects exactly one row per patient, with the patient index in order.

It would be nice if the input format is more permissive. To check if a patient already exists, we could use as convention that always a mapping with source SUBJ_ID must be present. Other mappings will then be replaced such: all existing mappings deleted, mappings from the patient_mapping.tsv file added.