Support dimension scales / labels / coordinates

[rly]

In order to support dimension scales / dimension labels / xarray-style coordinates in the schema, we need a way to provide more information about each dimension and how it links to other named datasets in the group. See a very old proof of concept in spec.py in hdmf-dev/hdmf#192. In that PR, the dims key is expanded beyond a simple list of names or list of lists of name options, and a new coords key is added. Although the specifics of that PR will be completely revamped after updating to the latest version of HDMF, I am creating this issue and linking to that PR to track ongoing work and milestones. This enhancement to the HDMF schema language should be backwards-compatible and will target hdmf-schema-language 3.1.0.