Add Elya detector

This is a collaborative pull request to integrate the Elya detector system. As far as I know these are the user-facing parameters:

- Choice of one out of three cameras.
- Choice of one out of x filters
- Magnification

This leads already to several questions and requirements from the base:

- [ ] Is there more?
- [ ] How do we specify abstract choices? Currently, I use a fake `Selection` parameter that takes a list of things that it could be set.
- [ ] Is the magnification a range or a discrete selection? For the range case we obviously need a `Range` parameter.
- [ ] Is it a good idea to have a pointer to actual camera classes inside the detector?

Waiting for input from @sensej, Tomy and David.