cmake: cleanup, build type for nf2ff

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
engine interface: improve interpolation for e-fields on boundary edges

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
nf2ff: add boundary mirroring

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
nf2ff: remove some debug code

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>