xyst

Fixup ChoCG solve

This major change appears to produce a correct result for the potential
flow around a sphere, comparing well to the analytic solution.

The implementation follows

Rainald Löhner, Chi Yang, Juan Cebral, Fernando Camelli, Orlando Soto,
Jacob Waltz, Improving the speed and accuracy of projection-type
incompressible flow solvers, Computer Methods in Applied Mechanics and
Engineering, Volume 195, Issues 23–24, 2006, Pages 3087-3109,
https://doi.org/10.1016/j.cma.2004.07.058.

and features:

* tetrahedra-only elements
* linear finite elements
* explicit time stepping (for now)
* edge-based finite element operators
* tetrahedron and triangle super-edges
* Taylor-Galerkin stabilization for advection
* fourth order damping for the divergence constraint
* projection based on the pressure increment
* conjugrate gradients for the pressure

Works in parallel with asynchronous communication.