Add unit test to remove divergence

[apradhana]

Given a flag grid denoting pressure DOFs with the following convention:

• 0 Neumann pressure, meaning Dirichlet velocity on its normal face.
• 1 interior pressure dofs.
• 4 Dirichlet pressure. It means that it's not a solid collider, i.e. an open channel.

Remove the divergence of a MAC-grid velocity field.