Manually load data in AirNow reader to avoid pandas error

[lewisblake]

Designed to close #890

I can't imagine this is the most elegant solution, but it will take care of the new encoding issues in with pandas.read_csv(). It would be worth investigating the performance benefit of putting this in a try-except block where we try pandas, but if that fails, we use this method instead.

[codecov]

Codecov Report

Patch coverage: 93.33% and no project coverage change.

Comparison is base (2104317) 80.42% compared to head (537933a) 80.43%.
Report is 1 commits behind head on main-dev.

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev     #893   +/-   ##
=========================================
  Coverage     80.42%   80.43%           
=========================================
  Files           101      101           
  Lines         17433    17446   +13     
=========================================
+ Hits          14021    14033   +12     
- Misses         3412     3413    +1     

Flag	Coverage Δ
unittests	80.43% <93.33%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
pyaerocom/io/read_airnow.py	97.71% <93.33%> (-0.44%)	⬇️

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