Rename source term NS subsection

[lpsaavedra]

Description of the problem

The subsection's name to specify the source term of the Navier-Stokes equations in the parameter file was set to be xyz.

Description of the solution

The subsection was renamed to navier stokes, since there are source terms for other physics where the subsection has the physics name, for example, subsection heat transfer. The change had to be made in all the parameter files of the examples, the tests and the documentation.

[blaisb]

@lpsaavedra I think it's a good idea to change the name, but the physics name used elsewhere for the fluid dynamics is "fluid dynamics" and not navier-stokes. I think it would be a better idea to used fluid dynamics because this source term is also used in the VANS equations for example

[lpsaavedra]

@blaisb that makes a lot of sense. I have now updated the name of the section to fluid dynamics. I also found some parts in the documentation (multiphysics examples only) were the "enable" parameter was mentioned in the source term section. However, this parameter was deleted a while ago, so I have corrected that as well.