add rotation

[zingale]

This is a long-term project (probably requiring a postdoc).

Maestro uses a 1-d base state to represent HSE and evolves the departures from this. For a rapidly rotating star, this will breakdown -- there is not a 1-d HSE model.

Instead we will need to do a 2-d base state, r, theta, that describes the structure. Generating the initial model itself will be challenging, but a relaxation technique and perhaps based on a self-consistent field method as done with binary pairs (that is partially implemented in Castro) would work.

An idea is that we initialize a spherical star, evolve a step, reaverage the Cartesian state to r, theta, resetting the base state, and then evolve again. Once equilibrium is reached, we reset the calculation with this initial calculation and turn on the reactions, etc. to play the simulation forward.

The change to the code would be rather invasive -- we will need to change all references to the base state to take a 2-d array. It is not clear how to extend the current averaging method to this case (although it may actually be simpler), and it is not clear how to include a w_0 term since there will no longer be a simple elliptic equation that drives the expansion.

The second complication is that the constraint itself will not be able to be written as a projectionable operation. Since there will be no equivalent to beta_0 that can hide the density / pressure stratification inside the divergence constraint. This is similar to the issue discussed in Klein & Paulius 2012 for the general EOS (that we get around with an average gamma1 with perhaps a lagging). SDC might be a path forward here, using a lagged U in the grad p0 term now. But this will require mathematical analysis.

[zingale]

some refs:

https://arxiv.org/abs/1509.08609