Allow librarians to import MARC data from other libraries

When entering new books or editing existing books, I often have to manually copy from libraries that offer a MARC record for download. It would be great if I could directly import this data instead of having to typing it.

As an example, take _Das Postwesen im Postamtbezirk Buxtehude_.
This book exists as a really low quality import on Open Library at [OL26425107W](https://openlibrary.org/works/OL26425107W/)
The Deutsche Nationalbibliothek offers most of the lacking information on [their website](https://d-nb.info/1246029650). It offers downloads as [MARC21-XML](http://d-nb.info/1246029650/about/marcxml) and [RDF (Turtle)](http://d-nb.info/1246029650/about/lds).

The DNB is not the only national libraries offering this, even though the formats differ between libraries. The Bibliothèque nationale de France offers Intermarc and Unimarc instead, for instance. LIBRIS (National Library of Sweden) offers MARC21.

It would save me time and prevent spelling errors if I could import those datasets.

### Describe the problem that you'd like solved

A way to import MARC records from National Libraries, to at least improve existing records, but ideally also to create new books.

### Proposal & Constraints

As far as I understood, Open Library already imports MARC records from some libraries. At least I often read "imported by MARC record from library of ..." at the bottom of editions.

The import should not be more annoying than typing the stuff in manually. Also, there seems to be a lot of technical differences between different MARC versions - I probably won't be able to get up to speed in all of them, this would have to be handled automatically.

### Additional context

### Stakeholders

@hornc