BlitzGateway: Support linking annotations to other annotations

[DavidStirling]

A user came across this issue when trying to programatically create TagAnnotations linked under a parent TagSet.

Currently the linkAnnotation function on the Blitz wrapper classes uses the OMERO_CLASS class variable to construct the link object name. Since Annotation wrapper subclasses do not define this variable the function errors out searching for NoneAnnotationLink.

It looks like setting OMERO_CLASS = "Annotation" would fix this particular scenario (we need an AnnotationAnnotationLink), but it's not clear to me if this would cause problems with other functionality in the wrapper.

[will-moore]

It makes sense that linkAnnotation() should work to create AnnotationAnnotationLinks.

Just searching for OMERO_CLASS finds this code that would need checking/updating:
https://github.com/ome/omero-py/blob/master/src/omero/gateway/__init__.py#L3421

I seem to remember the Parent-Child direction of links in TagSet->Tag was not intuitive, but I may have this wrong.
Is it tagset.linkAnnotation(tag) or would it be tag.linkAnnotation(tagset)?