Improve test and example scripts.

- exactV.py uses the correct Mx value
- more useful -extra_vars in test scripts
Bigger formulas in the source code browser.
ssa_test_cfbc improvements

- write SSA exact solution in m/year
- save nuH to the output file
Inline IceFlowLaw::effective_viscosity().

It is called many, many times. It should probably be inlined.
Also: pre-compute the exponent in the definition of viscosity.
Add the taub (taub_x, taub_y) diagnostic.

This is the basal shear stress $\tau_b = - \beta \vec U$ that uses U
from the SSA stress balance. (This is for technical reasons. It should
use the 2D advective velocity computed by *any* shallow stress balance.)
New way of computing nuH.

We need to be more careful near ice margins. (Before this commit nuH
values near ice margins depended on velocity values in some neighboring
ice-free cells. When CFBC was "on", PISM sets velocity to 0 in ice-free
areas, and nuH should not depend on this choice.)

Also:
- Update the ssa_test_cfbc error report.

- Adjust the ice hardness computation. Now we treat margins the same way
  in SSAFD::compute_nuH_staggered_cfbc() and in SSAFD::compute_hardav_staggered().
Fix a bug in IceModelVec::setValColumnPL().
Re-wrote IceModelVec3::getValColumnQUAD().
Clean up and minor optimizations.

- Update columnSystemCtx::solveTridiagonalSystem().
- There's no need to use PetscScalar **x (PetscScalar *x will do).
- Optimize PISMStressBalance::compute_volumetric_strain_heating().
- Precompute 1/n in IceFlowLaw.
- Clean up in SSAFD.
Add new SSA CFBC notes, including ideas re. melange back-pressure.