Fix boundary conditions after DG optimisation

This MR fixes issues from the DG optimisation introduced in MR !555 (merged) inside the compressible flow solver, whereby MPI deadlocks under the use of most boundary conditions due to the use of ExtractTracePhys inside the boundary condition evaluation. This is fixed by pulling these calls out to the parent function and passing them through to the boundary condition functions, which is in fact more efficient and avoids the use of these functions repeatedly for each RHS evaluation.