not test (in opt mode).

added a way to compute at each time step the mean flux
and the area of a section in the NS solver.
A first pre-process is done to get the faces that live
on the sought sections (search on points).
Then a special post-process is performed (it is called at each time step).

This function heavily depends on the mesh : OK for cylinders!

added test_levelset_ipstab in the list of SUBDIRS

A little modification: now the user must specify the getpot section for solver data. This is particularly useful when more than one solver is used at the same time, since it allows for a more cunning organization of the data file.

Test level set solver modified to incorporate modifications in HyperbolicSolverIP and LevelSetSolver classes.