Side of the box in which plane waves are evaluated on the boundaries: negative would mean
on boundaries which are towards negative values in the selected directions (e.g. -x).
maxwell/maxwell_boundary_op.F90 : 965
call parse_variable(namespace, 'PlaneWavesSide', MXLL_PLANE_WAVES_NEGATIVE_SIDE, bc%plane_wave%side_of_the_box)