Add option for repeating boundary conditions

[hvbish]

Requesting an option to enable repeating boundary conditions in the model grid for selected axes.

For example: a photon that leaves the grid through any face of the grid cube perpendicular to the xy-plane should re-enter through the opposite face of the cube, with the same weight and direction. If the photon leaves the grid though a cube face parallel to the xy-plane, the model grid ends as usual and the photon is no longer tracked.

An additional useful feature would be the option to randomize the direction from which the photon re-enters the cube, while preserving the position along the z-axis and the angle of motion relative to the xy-plane.