provide aerosol data via pooch

[mathause]

• xref ship some data with mesmer? #222
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

This has mesmer download and cache the aerosol data using pooch. I went for pooch because I have a bit of experience with it from regionmask. I think this approach should be fine, certainly as long as we stay in the MB range. pooch is added as additional dependency (however, regionmask also has pooch in its list of requirements so it's not really adding anything).

I expect the tests to fail because the data file is currently not yet available at the specified location on github. Maybe I do another PR for this.

[codecov]

Codecov Report

Patch coverage: 88.23% and project coverage change: +0.24 🎉

Comparison is base (de55522) 85.87% compared to head (dfd6156) 86.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #267      +/-   ##
==========================================
+ Coverage   85.87%   86.11%   +0.24%     
==========================================
  Files          38       39       +1     
  Lines        1515     1527      +12     
==========================================
+ Hits         1301     1315      +14     
+ Misses        214      212       -2     

Flag	Coverage Δ
unittests	86.11% <88.23%> (+0.24%)	⬆️

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

Impacted Files	Coverage Δ
mesmer/calibrate_mesmer/train_gt.py	91.46% <80.00%> (-1.04%)	⬇️
mesmer/io/load_obs.py	32.07% <83.33%> (+3.50%)	⬆️
mesmer/core/_data.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[mathause]

This now passes! Depending on #269 this should allow to remove cfg.obs_dir.

[mathause]

pinging @znicholls if you are interested to take a look

[znicholls]

Nice

[mathause]

thanks!