Origin-spanning primers

[manulera]

Hi @tnrich, I realised that those primers with text are not displayed properly if they span the origin. As a first step, I have changed getStructuredBases to handle origin-spanning primers, and added some tests in a first commit.

However, for the display I don't seem to understand how to split the primer text. I see that what I would have to do is somewhere in StackedAnnotations and has to do with baseEl, which is the span with the basepairs, but I don't understand how each part of the splitted annotation is rendered. As is, it renders like this: