make the AERONET reading more fault tolerable; add possibility to read gzipped files

[jgriesfeller]

Basically a new try for a fix of #518 done because I messed up the branch after @avaldebe merged the source run through black and isort.
No different functionality compared to #527

[codecov]

Codecov Report

Merging #538 (947837e) into main-dev (c906508) will increase coverage by 0.09%.
The diff coverage is 89.65%.

❗ Current head 947837e differs from pull request most recent head 9f9ad1a. Consider uploading reports for the commit 9f9ad1a to get more accurate results

@@             Coverage Diff              @@
##           main-dev     #538      +/-   ##
============================================
+ Coverage     76.69%   76.78%   +0.09%     
============================================
  Files            97       97              
  Lines         17494    17525      +31     
============================================
+ Hits          13417    13457      +40     
+ Misses         4077     4068       -9     

Flag	Coverage Δ
unittests	76.78% <89.65%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
pyaerocom/access_testdata.py	78.94% <ø> (ø)
pyaerocom/io/readaeronetbase.py	71.42% <88.88%> (+5.94%)	⬆️
pyaerocom/io/read_aeronet_sunv3.py	90.44% <89.47%> (-0.64%)	⬇️
pyaerocom/exceptions.py	100.00% <100.00%> (ø)

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[avaldebe]

I only have small suggestions, mostly a test that raises the new exception.
You can go ahead and merge if you think I'm nitpicking.

[avaldebe]

👍 ready to merge