Fix a bug in Equilibrium3D EoS

[isaaclegred]

Proposed changes

There was a "bug" (a mistake) in the previous version of the sound speed calculation involved in the promotion of 2-d to 3-d EoSs. The previous version may have been right but if it was I have no idea why. Essentially the wrong variable was being held constant in a partial derivative. I have:

1. Added docs the EoS class to explicitly clarify what needs to be computed in terms of variables that we use elsewhere
2. Fixed the thing that was being computed incorrectly, it is now computed correctly
3. Changed the test to use an IdealFluid EoS where the sound speed can be computed "by hand"

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

Someone should probably check the math in the docs, (thermodynamics is hard :( )

[nilsdeppe]

Thanks for doing this! A few comments on the documentation that you can squash directly :)

[isaaclegred]

@nilsdeppe this is updated when you get the chance!

[nilsdeppe]

Awesome! Thank you very much for doing this :D