Fix Version/s: None
Improve integration of lsst.verify.Measurement into the new lsst.verify API design.
- Make the "check" method against Specifications consistent with Specification.check().
- Make "extras" persist into a per-measurement Blob instead of an adhoc measurement data container.
- Review serialization and deserialization.
- Make MeasurementSet consistent with MetricSet and SpecificationSet.
- Refine the story for output_measurements() given the full API.