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%.

This commit adds stack-structured timing code, such that bits of SPINS
can be instrumented with

>  timing_push("short name")
>  [... work goes here]
>  timing_pop()

This is recursive, such that already-timed portions of the code can call
the timing code to further instrument subroutines or discrete code
segments, and the resulting structure follows the -dynamic- call stack.
This does have the odd result that portions of the code that are re-used
(namely spectral derivatives) can show up as several 'leaf' nodes in the
graph.

To print out the report to standard error, call

>  timing_stack_report()

The exact format of the report should be considered tentative.

and write vertical chain was moved to BaseCase (can write out vertical profile at a high temporal frequency)

Safety dump estimates output write time and writes fields with enough time before the computation limit is reached