Merge readgroups and readsets

This is a hopefully straightforward/non-controversial first step of step 1 in #32.

This change merges the two objects:
- removed `comments` in favor of more generic `tags` like on Reads (where comments can be put easily)
- made the id field non-optional
- started adding descriptive text to each field

I purposefully avoided:
- clarifying what a readgroupid is
- changing any of the other readgroup fields to be more restrictive/or eliminated/collapsed into tags/etc
- any other metadata work