Add Wall_t struct; mod initialize(), render().

particle_collisions/src/particle_collisions.c
	-Introduce a `Wall_t` struct, to represent wall objects, and a
	global `g_walls` (analogous to `g_particles`) to contain them.
	-Add statements to `initialize()` and `render()` to respectively
	initialize `g_walls` and render them.

particle_collisions/src/screen.(c, h)
	-Implement and document `drawLine()`, for `Wall_t` rendering.
Replace lead-asterisks in block comments with tabs

particle_collisions/src/(particle_collisions.c, screen.h)
	-Replace leading asterisks in block-comments on lines between
	the opening and closing delimiters (`/*`, `*/`) with hard-tabs,
	in compliance with my new comment format.
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.
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()`.
Merge branch 'feature/particle_collisions.walls' into feature/particle_collisions

particle_collisions/src/(particle_collisions.(c, h), geometry.(c, h))
	-Implement `Wall_t` wall objects, collision detection between
	particles and walls, and reflection off walls.