Test reading all reference files

[embray]

The existing tests of the reference files only read the v1.6.0 reference files--should try testing all of them going back as far as possible on previous versions.

Right now some simple things I believe will fail, but can easily be corrected once there are some tests written. Should also ready the data blocks and verify their checksums.

[embray]

Actually all the reference files are just #ASDF 1.0.0 anyways so structurally reading them is not an issue. It's more a question of making sure all the different tag versions of the core schemas are supported. This will also require making progress on #42 , which is important anyhow.