Incorporate a vertical advection scheme into Aether

[aaronjridley]

Description

Addresses #25
Addresses #24
Addresses #14

I have ported the Rusanov vertical solver from GITM into Aether (for the neutrals only at this point). In order to do this, I had to: (1) calculate the time-step; (2) improve the vertical boundary conditions.

Type of change

• New feature - this is a new vertical solver for Aether.
• Documentation - I have added some text in the overleaf document.

How Has This Been Tested?

A test is included in the tests/vertical_rusanov directory. This was run on the Mac and on Linux.

Because of the non-uniformity in the post processor, the script has to be edited to point to the right plotter.

Test configuration

• Operating system: Mac OS 13, Linux Ubuntu
• Compiler, version number: gcc versions 12 and 11
• Any details about your local setup that are relevant - no fortran, no netcdf

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• [N/A] Add a note to CHANGELOG.md, summarizing the changes