matlab: allow empty directions in field interpolation

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
tutorials: update to simple patch antenna tutorial

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
pml: fix in material calculation

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
operator: make Grid_Path a typedef struct

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

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