Finished (I think) implementation of half-edge in the Mesh class

Note that, for now, the parseGeometryDescription() function in MyGLWidget (which handles parsing descriptions of and constructing procedural geometry) is commented out, because it won't build until it's updated to work with the new half-edge structure.
Merge branch 'Half-edge'

Conflicts:
	RayTracer/Program1/Mesh.h (resolved)