Implement average exposure effect

[oczoske]

Scopesim has always produced output that was the sum over ndit subexposures. Many/most instruments produce averages, and this will likely be the case for METIS and MICADO. This PR implements an effect ExposureOutput, which takes an argument mode, which can be either average or sum. It should come after the detector noise effects, all of which assume a summed exposure. Unfortunately, this means that currently both SummedExposure (before the detector noises) and ExposureOutput need to be in effects chain. Since the former has no user-settable parameters it might actually be integrated into the readout() method and disappear from view.
I couldn't resist giving the effect a set_mode() method for user convenience.

So far, the effect is only set in METIS img_lm, cf. AstarVienna/irdb#224

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.76%. Comparing base (6cb80a7) to head (cc68b3f).
Report is 269 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
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+ Coverage   76.68%   76.76%   +0.07%     
==========================================
  Files          69       69              
  Lines        8536     8563      +27     
==========================================
+ Hits         6546     6573      +27     
  Misses       1990     1990              

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[teutoburg]

Seems good for now 👍