Fix lat-to-lon aspect ratio (now for real)

[mherrmann3]

As noted in #125 (comment), the calculation of the aspect ratio for the 'fast' projection plotting option was still not correct. This PR fixes this.

To summarize:

• original (issue Missing region border when specifying a projection; In addition: a (faster) alternative to using projections #109): 1 / (LATKM / 111.320 * np.cos(np.deg2rad(lat_region))) -- WRONG (but almost correct)
• simplification (PR missing region border in plots #110): 111.320 * np.cos(np.deg2rad(lat_region)) / LATKM -- VERY WRONG
• simplification (PR Fix simplification of lat-to-lon aspect ratio calculation #125): 111.320 / LATKM / np.cos(np.deg2rad(lat_region)) -- WRONG (but almost correct)
• correct (this PR) LATKM / (111.320 * np.cos(np.deg2rad(lat_region)))

Apologies for the multiple confusions! Next time, I will do the maths first... 😉

[codecov-commenter]

Codecov Report

Merging #127 (6583bcb) into master (e2bfd9b) will not change coverage.
The diff coverage is n/a.

❗ Current head 6583bcb differs from pull request most recent head 97a5715. Consider uploading reports for the commit 97a5715 to get more accurate results

@@           Coverage Diff           @@
##           master     #127   +/-   ##
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  Coverage   57.12%   57.12%           
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  Files          19       19           
  Lines        3142     3142           
  Branches      452      452           
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  Hits         1795     1795           
  Misses       1237     1237           
  Partials      110      110           

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[wsavran]

looks good @mherrmann3 ! i can take partial responsibility for missing this as well. ive checked this and it now looks ok. merging this PR!