`FilterWheelList` effect

[teutoburg]

I thought there was already an issue for this, but it seems that was just in my head...

Anyway, point of this is mostly that MICADO has two filter wheels (mirroring the physical instrument), which creates two main ... complications:

1. Especially for new users, it can be a challenge to figure out that their desired filter may not be in the effect that the tutorials mention (I know this happened to myself initially).
2. The pipeline expects a single filter name in the header (see the discussion in MICADO: crude additions to IRDB for FITS header keywords irdb#278).

This means we need some kind of entity in ScopeSim to manage both setting (1.) and getting (2.) the current filter in the correct filter wheel. The cleanest way to do this would be a container effect that hold the two individual filter wheel effects. This would still allow an advanced user to set a funny combination of more than one filter in the optical path by modifying the filter wheels' filters directly, in which case the "manager effect" should probably emit a warning about the value it writes in the header (or rather when the FITS header effect queries it for that...).