Support backwards compatibility for tag schemas

[drdavella]

There doesn't appear to be a mechanism to support backwards compatibility for tag schemas. For example, although different versions of the standard are allowed, the versions do not appear to have any effect on the tag schema versions that are used.

For example, maybe hypothetical version 1.5.0 of the standard provides support for version 1.1.0 of wcs/frame, and version 1.6.0 of the standard provides support for version 2.0.0 of wcs/frame. Even if the library provides support for both versions of the standard, there appears to be no way for version 1.1.0 of wcs/frame to be used (this example ignores the question of how schema and standard versions should be correlated).

A possibly relevant observation is that the library's version_map only appears to be used when writing new asdf files, but not when reading or creating new AsdfFile objects. It will probably be necessary to make use of this information in order to support backwards compatibility for tag schemas.

[drdavella]

This has been resolved by PR #272.