Clean up unit_tests, implement coordinate macros.

Add:
	src/unit_tests.(c, h)
		-clean up file with some documentation, function re-organization (so as
		to correspond to the order of function declarations in src/matrix.c).

	src/matrix.c
		-substitute occurrences of array indices 0, 1, 2, and 3 with with macros
		X, Y, Z, and W respectively.