For Calibration Mappings in CameraMapper, in New Butler repositories, the Calib Mapping needs access to another registry.

For this, implement Repository Relationships.

• Butler needs a way to “relate” registries from 2 input repositories so that metadata lookup can be done in one and then completed in the other.
  *The relationship between the calib repository and the raw repository’s registry must be recorded in the persisted cfg of the output repository. (It must be in the output repository’s cfg because the relationship does not affect the contents of any of the input repositories, but it does affect the contents of the output repo, so that is where it should be stored).
• A typical use case example is for a repo containing Raw images and a repo containing Calib images: For a given exposure number, look up the exposure date+time in the raw registry, and then in the Calib repo use that date+time to find the dataId for the Calib image that was valid for that date+time.