Add error message for overlapping elliptical apertures

[cemitch99]

The case of a periodic array of elliptical apertures yields incorrect results when the apertures overlap, which occurs when xmax >= repeat_x/2 or ymax >= repeat_y/2. This case is also unphysical, since the solid material of the aperture plane is not topologically connected in this case.

Check for this condition, and add an error message.

The case of overlapping rectangular apertures simply yields full transmission, and no error message is required.

Pointed out by SchroederSa.

[cemitch99]