allow for database detector

[shallmann]

Enhances the detector_viewer app by including the option to display the RNO-G detector (loaded from the mongo database).

[shallmann]

Note: see the documentation for how to start the application.

Will leave adding a display of devices for the future, since they are not yet accessible from the database it seems.

[sjoerd-bouma]

Thanks Steffen, sorry I missed this. I figured the time slider feature (which unfortunately is hardly usable anyway) should apply to the rnog_detector as much as the normal detector, so I made it work for both. I also fixed the merge conflict.