Add applyThresholdPressures() method and usage.

Note that this commit does not introduce any way to set
use_threshold_pressure to true, so the new code is not run.
Add setThresholdPressures() method.
Rename variable.

Changed threshold_pressures_by_face_ -> threshold_pressures_by_interior_face_.
Remove files added by mistake.
Change setThresholdPressures() interface.

Now it takes a std::vector instead of an Eigen::Array, more importantly
it expects one element per face, not interior face. The mapping now takes
place in setThresholdPressures().
Pass dummy threshold pressures through simulator to solver.
Resize array before assigning elements.
Fix bug: exactly the wrong potentials were nullified.
Make simulator programs actually obtain threshold pressures.
Merge pull request #190 from atgeirr/threshold-pressure

Threshold pressure