Pressure constraint gmg

[blaisb]

Description of the problem

• When the mesh is not homogenous and the domain is closed (e.g. only dirichlet or periodic BCs), the coarse grid solver of the matrix-free GCMG or LSMG does not work. This is because the problem is ill-posed due to the pressure constant.

Description of the solution

• On the coarse grid level, fix a pressure DOF to be zero by adding a single constraint. This ensures that the solution works.

How Has This Been Tested?

• I changed the example to use this feature, but I also need to add an application test. Will do so in this PR.

Documentation

• New parameter is updated and documented.

Comments

• I am glad this is fixed.

[blaisb]

@lpsaavedra we should merge this after your PR is merged. We just need to regenerate the output. I made it from my mac and it seems to lead to problems