MNI Transformation tools

A while ago I wrote these functions for finding linear subject-MNI transforms and transforming data to and from MNI space. They seem like they might be pretty useful for people, so I'm adding them to pycortex. 

(Also, I selfishly wanted to be able to cache the results, and doing that through the pycortex db seemed right.)

Any objections?

Err, and there's one vague issue outstanding: the default template that I've specified is the nonlinear MNI 152 template that's included in FSL. That might be non-ideal for this linear transformation, but it seems to work just fine in practice.