FULL MPI

[edoddridge]

Aronnax can now use MPI to run on multiple processors. Closes #47

I have only benchmarked on my laptop, but going from one core to two cores increased the speed by 65%. While there's room for improvement, this is still excellent.

There are currently far too many exchange calls. These were required to keep the results identical to previous single-core results which wrapped fields far too often. Judicious use of the halos should remove the need for this, and will be addressed in the future. These calls will likely limit the speed up that can be obtained with large numbers (O(10s-100s)) of processors on big clusters.

All previous tests pass. In addition to this, there are many new multicore tests:

• some that run the same simulation on different tile decompositions and ensure that the results do not change;
• some that take single core simulations and restart them as multicore simulations.

Once merged this is worthy of a new release! 0.3.0 here we come.

[codecov[bot]]

Codecov Report

Merging #212 into master will increase coverage by 0.68%. The diff coverage is 95.21%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
+ Coverage   91.14%   91.82%   +0.68%     
==========================================
  Files          19       20       +1     
  Lines        1603     1822     +219     
  Branches       82       96      +14     
==========================================
+ Hits         1461     1673     +212     
- Misses        116      124       +8     
+ Partials       26       25       -1

Impacted Files	Coverage Δ
src/bernoulli.f90	100% <100%> (ø)	:arrow_up:
src/advection_schemes.f90	100% <100%> (ø)	:arrow_up:
src/exchange.f90	100% <100%> (ø)
src/state_deriv.f90	100% <100%> (+8.33%)	:arrow_up:
src/aronnax.f90	94.18% <100%> (+4.18%)	:arrow_up:
src/momentum.f90	92.24% <100%> (+0.06%)	:arrow_up:
src/enforce_thickness.f90	70% <100%> (ø)	:arrow_up:
aronnax/driver.py	93.85% <100%> (+0.16%)	:arrow_up:
src/end_run.f90	58.82% <100%> (ø)	:arrow_up:
src/thickness.f90	97.67% <100%> (+0.02%)	:arrow_up:
... and 9 more

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