Conversion from Records to DMPs

[max-moser]

One side of the integration (conversion from DMPs to records/drafts) is already implemented.
The other side of the integration (conversion from records to DMPs) still needs to be implemented.

This requires additional methods in the BaseRecordConverter, as well as some frame logic similar to convert_dmp.

Since the scope of allowed changes to DMPs made by the RDM repository is mostly limited to the distribution level (except for the addition of new dataset_identifiers and metadata schemas on the dataset level) and the update semantics is PATCH (i.e. only the supplied values are subject to change), the frame logic will likely be relatively simple.