Spurious warnings about non-existing boundary ids with prescribed_regions

[Stephan Kramer]

When using the option prescribed_regions under Velocity, spurious warning messages are written to fluidity.err-0 about boundary conditions for the scalar fields associated with surface ids that are not part of the prognostic region. This is caused by populate state being called on the 'substate', i.e. the state that is derived from meshes with the prescribed regions removed.

Maybe we can pass some logical to populate_boundary_conditions (passed on to add_scalar/vector_boundary_condition) to supress warnings when called form populate_sub_state?