Implement validation checks on records for unconditionally required fields

[jamiewhths]

When importing records via the R package, fields which are marked as "unconditionally" required in the form schema -- that is, there are no other relevance rules set on the field -- should be enforced before importing into ActivityInfo. This is a small measure which should help to improve data quality on records imported via R scripts.

[nickdickinson]

Do we want to add this to a milestone? @jamiewhths