Open_mfdatarray bug fix

[edoddridge]

The function open_mfdataarray didn't respect the location of the variables on the grid. This PR introduces a test that can detect that behaviour, and a fix for the problem.

[codecov]

Codecov Report

Merging #208 into master will increase coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   90.44%   90.51%   +0.07%     
==========================================
  Files          19       19              
  Lines        1601     1613      +12     
  Branches       82       82              
==========================================
+ Hits         1448     1460      +12     
- Misses        126      127       +1     
+ Partials       27       26       -1

Impacted Files	Coverage Δ
aronnax/core.py	95.34% <87.5%> (-0.72%)	⬇️
src/model_main.f90	95.65% <0%> (+1.73%)	⬆️

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[edoddridge]

This pull request introduces 2 alerts when merging 630e6ef into 1487ff0 - view on LGTM.com

new alerts:

• 1 for Unreachable code
• 1 for Redundant assignment

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Comment posted by LGTM.com