Add AeroVal config documentation example

[jgliss]

This PR adds an AeroVal example configuration script in the ReadTheDocs page, that explains the main setup and options.

It also fixes a few issues that were detected after release of v0.12.1 when trying to run the example setup, e.g. #494.

[codecov]

Codecov Report

Merging #495 (851abb2) into main-dev (b3d15ac) will increase coverage by 0.03%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##           main-dev     #495      +/-   ##
============================================
+ Coverage     76.61%   76.64%   +0.03%     
============================================
  Files            97       97              
  Lines         17358    17357       -1     
============================================
+ Hits          13299    13304       +5     
+ Misses         4059     4053       -6     

Flag	Coverage Δ
unittests	76.64% <71.42%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyaerocom/aeroval/setupclasses.py	93.14% <ø> (ø)
pyaerocom/io/read_aeronet_invv3.py	34.04% <ø> (ø)
pyaerocom/io/read_aeronet_sdav3.py	92.78% <ø> (-0.08%)	⬇️
pyaerocom/io/readgridded.py	72.54% <ø> (ø)
pyaerocom/io/readaeronetbase.py	65.48% <52.94%> (+1.73%)	⬆️
pyaerocom/aeroval/coldatatojson_engine.py	85.29% <100.00%> (ø)
pyaerocom/colocation_auto.py	82.50% <100.00%> (+0.02%)	⬆️
pyaerocom/exceptions.py	100.00% <100.00%> (ø)
pyaerocom/io/read_aeronet_sunv3.py	91.07% <100.00%> (ø)
pyaerocom/stationdata.py	81.51% <100.00%> (ø)
... and 1 more

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[jgliss]

@dulte and @jgriesfeller and @AugustinMortier please be aware of the changes here, I found some bugs and inconsistencies in the variable handling for the output files when creating the example config. The problem is that pyaerocom was using the AeroCom variable name for the output files and not the input variable name, which resulted in AeroVal not displaying "ang4487csaer" from NorESM2 in the model maps plots, because "ang4487csaer" is an alias for "ang4487aer".

I think this affects only cases where alias variables are used, and it is good that this was detected now, since this variable mappings is always a mess.

[dulte]

Looks good :) Wonderful documentation of the config. I will use it to document a more advanced example :)