Add absorber option to Aperture element.

[cemitch99]

Added an additional (optional) string input to the Aperture element, allowing the user to select between two "actions":

transmit - the specified boundary describes the plane region in which particles are transmitted (default) absorb - the specified boundary describes the plane region in which particles are absorbed

The second option yields the "negative" of the first, and is useful in describing, e.g., absorber sections.

• [x] Add additional argument to Aperture element.
• [x] Update Python equivalent.
• [x] Add benchmark example.
• [x] Update documentation.
• [x] Address Issue #757

Overlapping elliptical apertures: