Add typedefs, geometry, lineSegmentsIntersection().

particle_collisions/src/geometry.(c, h)
	-Add a module to implement all of the linear-algebra and other
	geometric maths that the particle system requires.
	-Implement and document (the as of yet untested)
	`lineSegmentsIntersection()` and `crossProduct()`.

particle_collisions/src/particle_collisions.h
	-Add `Point_t` and `Vector_t` typedefs, for
	readability/self-documentation.