How to handle boundary conditions under MPI

[TomMelt]

Placeholder issue for discussion in Bergen

[einola]

Dynamics solves the problem on the whole domain, including the ghost cells. This involves "wasted computation", as the ghost cells are overwritten.

Open boundaries are handled by setting the values of the ghost cells outside the boundary to

• zeros (we'll start with this one)
• the value of the cells on the inside of the boundary
• values read in from files

Periodic boundaries are handled purely by copying values across between MPI ghost cells.

For the north-pole fold we need to flip the dg components. @winzerle will tell @TomMelt how to do this.