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.