Implement boundary conditions using ghost cells

[babrodtk]

Implement boundary conditions for cartesian grids.

• Reflective
• Periodic
• Constant
• ...?

Use the global boundary descriptors posX, negX, posY, negY, etc.

Reflective boundary conditions are somewhat special, as they require negating certain components of an equation (e.g., hv along the y-axis for the shallow water equations)

Periodic boundary conditions should preferably specify only which axis to be periodic about (e.g., it does not make sense to have periodic along negX and reflective along posX)