Generalized Aperture: Grid

[ax3l]

@SchroederSa raised a request for a generalized aperture element where we can repeat the current rectangle or ellipse acceptance by an offset X and Y on a grid.

We think a quick, efficient way to implement this:

• add optional repeat_x/y parameters to the aperture
• if specified, repeat the transmitted part periodically along X or Y or both (along a regular grid) as an infinite mask
• if a finite mask is needed, users can a single-hole aperture afterwards