import/export functionality of mcf to datacite

In academia/FAIR the [datacite](https://schema.datacite.org/) is a common metadata schema for articles and datasets. conversion of Datacite metadata to and from mcf would be interesting for the following use cases:

- Importing metadata from datacite is a good start for a new mcf (retrieve metadata from DOI by adding the [Accept:application/x-bibtex](https://stackoverflow.com/questions/10507049/get-metadata-from-doi) to the DOI request)
- Exporting mcf to datacite can help in the registration of new doi's and enable metadata harvesting by doi oriented catalogues (OAI-PMH in pycsw)

AS discussed on FOSS4G Prizren:
- Datacite reader could be added to OWSLib (so it can be used in pygeometa and pycsw)
- Datacite writer can be added to pygeometa and pycsw individually