Rewrite fdt_rotate_bvecs in Python

Our DTI prep script currently calls a bash script that applies a transformation matrix to b-vectors in a roundabout way by directly editing text-files. It would be good to re-write this in Python when there is a moment.

https://github.com/QTIM-Lab/qtim_tools/blob/master/qtim_tools/external/fdt_rotate_bvecs.sh
