InitFDTD: API change to key/value only

legacy format is supported

Signed-off-by: Thorsten Liebig <thorsten.liebig@gmx.de>
operator: alternative material averaging method

If constant cell material is activated, material probing is performed
only in the center of a primary cell.

This should improve and simplify SAR calculation if all materials
are assumed as constant within a primary YEE cell.

Usage from Matlab/Octave:
FDTD = InitFDTD('CellConstantMaterial',1);

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