First and most trivially, this change adds a few more timing push/pop
invocations relating to multigrid and especially to some of the MPI
synchronization steps that might cause processors to wait on each other.

More substantively, this change also adds a new tridiangonal solver
based on the bog-standard forwards/backwards elimination Thompson
algorithm (see Wikipedia).  This should be acceptable because the 1D
problems being solved themselves come from a 2D problem, so we don't
expect ill-conditioning; calling out to the GMRES banded solver was
surprisingly a computational bottleneck perhaps because of pivoting.

This change seems to decrease the line-solve time by about 80%, which in
turn decreases the overall runtime (tank_rho test case) by 40%.