Merge branch 'feature/IncNSOpt' into 'master'
Improve performance of incompressible solver This MR contains improvements to the performance of the incompressible solver (some of the changes might also help in other cases). I worked on these changes based on a profiling of the `Cyl.xml` example (2D, N=5, using both tris and quads). For this particular case (in serial), I got a reduction of computational cost close to 30%, although I suspect simulations with higher order or in 3D will get more modest improvements. The main changes are: - Use a virtual function to make UnsteadySystem skip the FwdTrans it performs after each time step, since this is completely redundant for the incompressible solver - Introduce a parameter to choose the frequency at which the solver checks for NaNs - Tweak the default value of the mdswitch parameter for the multilevelstaticcond, and also allow to specify it in the session file - Include PhysDeriv in a single direction in the Collections, since we need it to compute div(u) for the pressure forcing - Use symmetric matrices for the interior part of the static cond (whenever possible) - Several small changes to reduce overhead costs, especially in matrix vector multiplications. See merge request !645
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