#2714 list fields

[iamleeg]

This adds fields where you can supply multiple values (either from a list or free entry). Some observations:

1. in CSV/TSV download, the list is stored in a single field as a comma-separated string (csv gets the escaping correct in the downloaded file).
2. the interaction between a field being required and a list field currently means that the list is required (but can be empty). The other interpretation would be that the list is required to contain something. There isn't really enough guidance in the schema document to make a call either way on this, or on whether any field is required.
3. I discovered today that you can attach custom metadata to a dataclasses.Field and that's probably a better idea for the Document class than carrying around a separate collection of data_service.model.field.Field objects. That said, I won't have time now to make that change: I document it as a gift to any future bored developer :).