Allow reference of optical path for annotations measurements

[hackermd]

In case of intensity measurements for annotations, the microscopy image (and the optical path) need to be specified for the measurements. This PR adds allows including a reference to the images to which the measurements apply.

cc @dclunie

[hackermd]

This looks good to me except one minor error I spotted. Though if I remember correctly, we were waiting on a decision about changes to the standard?

We have submitted a Correction Proposal and are currently discussing it with the DICOM Standard Committee. cc @dclunie