Add 3D EoS test helper and fix bug in 2d random generators

[isaaclegred]

Proposed changes

This PR adds a EoS Test helper for 3-d EoSs, and fixes what I believe to have been a bug in the 2-d EoS test helpers where the specific energy random generators was not being used correctly.

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

The 3D EoS test helper is essentially a pared-down version of the 2D helper (because we require fewer thermodynamic functions to be implemented for the 3D EoSs) though we work in terms of randomly generated temperatures. In general, random tests should probably be implemented with respect to rest mass density, temperature, and electron fraction, rather than the specific internal energy, because it is hard to determine the valid range for the specific internal energy but it's typically very easy for the temperature.

[nilsdeppe]

Please go ahead and squash :) I have no idea how clang-tidy is deciding these things. It seems to be wrong for the header file...

[isaaclegred]

@nilsdeppe (possibly) modulo the clang-tidy errors I think this is ready to go