Astropy version awareness

[drdavella]

Issue #249 addressed the need to support backwards compatibility for tag schemas. These schemas will change over time for various reasons and it will probably be desirable to maintain some form of backwards compatibility.

Recently a change to astropy required changes to several of our schemas (this is discussed in issue #212 and is being addressed in PR #244). Since this change is tied to a particular implementation of astropy, once it is implemented, using previous versions of astropy will not work unless some form of backwards compatibility is provided. However it is not enough to simply provide backwards compatibility for multiple tag schemas: it is necessary to tie particular tag schema versions, and therefore particular versions of the ASDF standard, to specific astropy versions.

This makes an already complicated versioning schema even more complicated. It may be sufficient to have a high-level check in asdf that makes sure that the installed version of astropy is compatible with the currently supported asdf standard.

[drdavella]

I believe this issue has been sufficiently addressed by PR #244 and #272.