Red grav bot drag

[edoddridge]

Add bottom drag to reduced gravity simulations with an active lower layer.

Added code to allow this feature, and a test to make sure it works as expected.

[codecov[bot]]

Codecov Report

Merging #243 (f8459a4) into master (2424b32) will decrease coverage by 0.31%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   74.41%   74.10%   -0.32%     
==========================================
  Files          20       20              
  Lines        1419     1425       +6     
  Branches       85       89       +4     
==========================================
  Hits         1056     1056              
- Misses        313      315       +2     
- Partials       50       54       +4     

Impacted Files	Coverage Δ
src/momentum.f90	66.43% <40.00%> (-2.91%)	:arrow_down:
src/state_deriv.f90	80.00% <100.00%> (ø)

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[edoddridge]

The code coverage metrics are still wrong :( See #242 for details.

The new code is tested by a new test in physics_tests.py, since the effect of linear bottom drag can be easily predicted analytically.

Since all other tests pass, I'm going to merge this.