Etendue is calculated but ignored

[teutoburg]

The OpticsManager sets "!TEL.etendue". The only place where that's then used is in SurfaceList. Following its path:

• SurfaceList.emission needs "etendue" in self.meta otherwise throws an error; calls:
• SurfaceList.get_emission() which has an etendue argument, but that isn't actually used anywhere; calls:
• SurfaceList.combine_emissions() which mentions in the docstring: "The function assumes that etendue is conserved through the system, i.e. surfaces are neither over- nor undersized."

That last point might be problematic if that requirement isn't met, which is the case in MICADO calibration mode, where the relay mirror is slightly larger than the calib mirror, see also #997.

[oczoske]

The docstring of .combine_emissions() can be interpreted in the sense that what it is given in the surface lists represents the beam area at the surfaces rather than the physical size of the surfaces themselves. We often use the sizes as an approximation to the beam areas as the latter are not that easy to obtain. I'll check the rest at some point, but I'm always much in favour of using etendue rather than areas and solid angles as the latter change on the way through the optical system.