Allow to disable Navier-Stokes source term in MF

[lpsaavedra]

Description of the problem

The evaluation of the source term function, even if it was zero, was affecting the performance of the matrix-free application, as the evaluation needed to be executed every time we performed a vmult using the matrix-free operators.

Description of the solution

This PR adds a new enable parameter that allows to disable the source term explicitly in the matrix free application. It allows to reset the pointer of the forcing function in mf_navier_stokes.cc and consequently to check within the matrix-free operators if the pointer exists or not, to evaluate the source term function.

How Has This Been Tested?

• All the tests pass and theenable = false parameter was added to all the matrix-free tests that do not require a source term.

Documentation

• The parameter was added to the source term page in the documentation with a tip that specifies when it is useful to explicitly disable the source term.

Future changes

• The Navier-Stokes source term parameter subsection needs to be renamed from xyz to navier stokes. Since this changes more than 100 files (tests, examples, documentation), I will do this in the next PR.

[blaisb]

@lpsaavedra you have a merge conflict now since I merged the other PR. Can you rebase?

[lpsaavedra]

I have rebased now @blaisb