tckmap dimensionality

So if someone could go over this line-by-line and make sure there's no mistakes, that'd be great :-P

Main change is the fact that tckmap can now map streamlines to a greyscale image, directionally-encoded colour, dixels, or TOD; all with full TWI functionality. Longer-term, the back-end changes will allow me to incorporate fixels into the mix.

I've tested fairly standard permutations (i.e. one or two variables at a time), think the only way that any bugs in obscure use cases are going to turn up is by merging; even then, it's even more modular than before, so things _should_ behave as expected.