Specify the null space for the Poisson system.

 * Let PETSc's solvers handle the singular system..
 * Do not modify any elements of QTBNQ.
 * Currently works only for NavierStokesSolver.
Minor edits.

 * Add PetIBM.cpp as a dependency so that it is recompiled when edited.
 * Change the size of the memtest case mesh to 4x4.
 * Destroy the viewer after writing the performance summary.
Specify null space in TairaColoniusSolver.

 * Add file TairaColonius/setNullspace.inl.
 * Make nsp local to the setNullSpace function.
 * Create temp and nullSpaceVec inside TairaColoniusSolver::createVecs.
 * Set default values for initialPerturbation.
Merge branch 'solveSingularSystem' into develop
Rename temp to regularizedForce.