previously we modeled gravity as a mixed formulation. However, this turns out to not be needed with the improved surface prescription. Removing this and returning to a simpler gravity system dramatically reduces the number of unknowns and therefore speeds up the system.
This is a class which adds some helper utilities for computing the effective potential and the hessian of a body under rigid rotation. Note this is not an equation in and of itself
added equation that sums pressure, gravity, and rotation together to get the displacement force. Note that some of the dependencies of this have not been commited yet as they lack documentation
These help constrain what are valid equations at compile time thereby preventing invalid equations from causing runtime crashes due to missing methods and members (this does not validate anythinng about the physics of an equation, only that the form of the equation is right). Note that this is done at compile time rather than with runtime polymorphism to avoid both indirection cost but more imporantly to allow the compiler to more easily inline functions and preform other optimizations that indirection can hide.
This equation provides a constraint on central density through specific enthalpy (I think this is general so long as the eos can convert, though I wrote this for the barotropic case). This may need to be changed latter to be more general (though also generally we should not constrain central density). this is really there to deal with the specific non-rotating n=3 case where the central density is ill-defined due to a singularity.
