Upgrade to JSON Schema draft-07

[perrygreenfield]

Users have expressed some interest in the new features of JSON Schema draft-07, so we might take this opportunity to designate draft-07 as the ASDF Standard 2.0.0 schema format.

One downside of this change is that draft-07 and draft-04 schemas are mutually incompatible, so all current schemas would need to be updated before they could be used with an ASDF Standard 2.0.0 file.

A potentially troublesome JSON Schema change introduced in draft-06 is that the "integer" type now validates any number with a zero fractional part, so floats like 1.0 will begin validating where they did not before.

Is this a problem since it is a relaxation and presumably won't break reading old files? It is a reasonable interpretation; but does it make it difficult to support in our or other libraries?

Actually it will help with at least one issue we know of in the jwst pipeline where a spectral order needs to validated as integer but it comes out of a model as a float because modeling turns everything into float.