Better BLOB handling

[rwb27]

It would be great to handle non-JSON outputs a bit better, in a few ways:

• Client-side support for specific media types - e.g. it will turn a dictionary with a mimetype and href into a Blob, and if the mimetype matches image/jpeg it might provide a subclass with methods to create an Image object.
• Consider minimising the need for BaseModel wrappers, instead use annotated types as we do for NArray
• Make it possible to return Blob types from properties (would need some logic around expiry)
• Managing calibration reports that must be retained for traceability