Add xform_brain, mirror_brain and brain2reg functions

[jefferis]

So after our discussion, I think the simplest strategy looks like:

brain2reg(reference=, sample=, mirror=)
xform_brain(x, reference=, sample=, ...) xform(x, brain2reg(refererence,sample), ...)
mirror_brain(x, brain=, ...) xform_brain(x, brain2reg(brain=mirror,...)

I think there could be some reasonable discussion about whether a mirror_brain function was also necessary, but people might expect to find one given the existence of nat::mirror.

[jdmanton]

Implementation started in cd76438.

[jdmanton]

Done in cd76438.