Remove arucos from particle landmark lists if the aruco is part of a wall.
No need to maintain separate mu/sigma info for wall arucos.
Should still allow single arucos to be used as landmarks if they
are not part of a weall definition.

This will speed up the particle filter because we won't be calling
process_landmark on all the wall arucos.


Intersection of two line segments needed for path planner:
https://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect
http://www.cs.swan.ac.uk/~cssimon/line_intersection.html