Decorate schema with specification version information

[jonnew]

I suggest that the JSON schema would be more useful if it was:

1. Annotated with the spec version it validates
2. Potentially versioned alongside spec releases (e.g., schema/0.2.21/probe.json.schema)

For versioning, one easy option is to add a comment:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$comment": "Validates the probeinterface specification version 0.3.2. See https://probeinterface.readthedocs.io",
....

This could be automated during the creation of a release.