[MICADO] Slit-through images

[wkausch]

We need the possibility to simulate slit-through images with MICADO. This works if the effect spectroscopic_slit_aperture is moved from MICADO_spec.yaml to MICADO.yaml and !OBS.slit_file is set to an existing mask file. However, we have not found a way to disable the slit by default, e.g. by setting !OBS.slit_file to false or none or something like that. METIS uses a slit wheel effect, which is enabled in imaging but with slit = false as default.

[oczoske]

It should certainly be possible and desirable to change the MICADO setup to follow the METIS model. It is less clear whether this is easily possible while still retaining the old modes for backward compatibility.