No tempfile

[avaldebe]

Address some of the problems behind #692 by providing temporary files as fixtures.
These changes makes possible to run multiple tests in parallel with pytest-xdist.

[codecov]

Codecov Report

Merging #694 (d29b3a5) into main-dev (e9f2666) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           main-dev     #694      +/-   ##
============================================
- Coverage     76.81%   76.79%   -0.03%     
============================================
  Files            97       97              
  Lines         17536    17536              
============================================
- Hits          13471    13466       -5     
- Misses         4065     4070       +5     

Flag	Coverage Δ
unittests	76.79% <ø> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
pyaerocom/io/iris_io.py	93.07% <0.00%> (-1.54%)	⬇️
pyaerocom/io/helpers.py	70.49% <0.00%> (-0.82%)	⬇️

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[avaldebe]

[...] These changes makes possible to run multiple tests in parallel with pytest-xdist.

Unfortunately, it does not work consistently on CI.