Finish sphere-wall collisions; add reflection.

particle_collisions/src/geometry.(c, h)
	-Add a `NO_INTERSECTION` global `const int`, for
	`lineSegmentsIntersection()`.
	-Implement `reflectVector()`, to allow the reflection of
	particles off walls.

particle_collisions/src/particle_collisions.c
	-Add `particleWallIntersection()`, a wrapper for
	`lineSegmentsIntersection()`.