[WIP] fluid pressure boundary conditions

[jdannberg]

This pull request adds the option to prescribe boundary conditions for the fluid pressure (and not just the gradient of the fluid pressure) in models with melt transport.

Things I still have to do:

• make sure we do not assemble the fluid pressure gradient boundary terms on boundaries where the fluid pressure is prescribed
• make an enum that lets plugins report if the can prescribed the fluid pressure, the fluid pressure gradient, or both, and check that the plugin selected and the type of boundary condition selected works together.