Add vertical diffusion

[edoddridge]

This PR implements a naive diffusive mass flux between layers, as discussed in #175. For most simulations the diffusion coefficient, kv, will be set to zero.

It also streamlines the names of the diffusive and viscous parameters, closing #176.

The new functionality is tested in two simulations:

• a 1.5 layer simulation in which the simulated layer thickness is compared to an analytical expectation; and
• a 3 layer simulation in which the thickness of the top and bottom layers is compared, to ensure that they grow at the same rate.

[codecov[bot]]

Codecov Report

Merging #177 into master will increase coverage by 0.62%. The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   94.98%   95.61%   +0.62%     
==========================================
  Files           5        5              
  Lines        1277     1299      +22     
  Branches       70       73       +3     
==========================================
+ Hits         1213     1242      +29     
+ Misses         47       41       -6     
+ Partials       17       16       -1

Impacted Files	Coverage Δ
aronnax/driver.py	93.45% <ø> (ø)	:arrow_up:
aronnax.f90	95.62% <96.66%> (+0.81%)	:arrow_up:

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