Switched velocity Helmholtz solves to 'hsolve' and enabled projection.

The p92, p94 flags now actually enable velocity projection.  Each
velocity component has its own approximation space of size p92.
It kicks in after p94.  If p92 <= 0 or p94 == 0, no projection
is used.

This also has the advantage of unifying the solver code base a bit,
removing the need for ophinv.