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 solo arucos to be used as landmarks if they are not
part of a wall definition.

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

**** Current bug: if we kidnap the robot so pf state is 'lost',
make_particles_from_landmarks is punting because it only looks for
solo aruco landmarks now; doesn't construct walls.


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Bug: If we see markers 43 and 44 we create Wall-43.  If we then pick
up the robot so it delocalizes, and put it down so that it only sees
marker 45, it will not add the marker or re-localize, even though
it should know where it is because marker 45 gives it its
position with respect to Wall-43.

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Should replace integer Aruco marker ids with 'Aruco-###' strings
throughout.

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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