Include a test planet - Acheron and allow 2D simulations for horizontal only testing

[aaronjridley]

Description

Working towards issue #90

In order to test the 2D cubesphere advection scheme, we want to limit several things in the code:

1. only allow horizontal processes (so 2d in an altitude slice)
2. only have 1 species to begin with, so total density = species 0 density
3. turn off a bunch of other source terms

We have added a new planet "acheron" that is O and O+ only with exactly 10 m/s2 gravity (@1000 km) and 1 AU circular orbit.

We have tested that this works in 3D and in 2D, and have a test for 3D acheron and 2D earth.

Type of change

• New feature - acheron as a planet and making sure 2d works.

How Has This Been Tested?

tests/2d_alt/run_test.sh

tests/acheron/run_test.sh

Test configuration

• Operating system: WSL!!! (Ubuntu)
• Compiler, version number: g++10

Checklist:

• [N/A] Make sure you are merging into the develop (not master) branch
• 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