Improve usability of builders in respect of lists

It's highly convenient when using the datamodel to create a new representation to be able to create the lists (e.g., of alternate identifiers and files) in the new IE piece by piece instead of having to make them _de novo_. This change does that; it adds methods that append single items to the lists.

It also makes it so that lists in builders are always present, rather than having values that are sometimes `null` and sometimes not.