Tracking coverage of OA entities

[yjunechoe]

New file openalexR-coverage.R tracks oa2df()'s coverage of OA entity fields (currently just one draft for the Work entity)

New function get_coverage() introduces an interface to the coverage information, given an entity

This could also use some tests to ensure a match between what oa2df() actually returns vs. what we say it returns.

[yjunechoe]

@trangdata Could you help me double check my understanding of how we process variables from Work objects? And feel free to suggest anything to add to the comments column

[trangdata]

Thank you for putting this together @yjunechoe 💯 We needed this a long time ago. You're as thorough as always! 🚀

I will make a PR if I think of other things to add to comments.

[rkrug]

I like this functionality.

One point: in the case of e.g. biblio, where there are values extracted, it would be great if the field could be specified, e.g. biblio.volume, volume and also biblio, NA to indicate where the value is coming from and that biblio itself is removed.

That would make it clearer to understand and also make it possible to rename sub fields after extraction.

[rkrug]

Oh - I would also suggest to add the sorting of the fields in this function (or a separate function).