ENH: i/o for fiducials fiff files
RF: invert_transform
DOC
ENH: create translation, rotation and scaling matrices
ENH: apply transform function
ENH: option to raise an error when get_subjects_dir() finds nothing
ENH: add MriHeadFitter
ENH geom: import mayavi locally
cleanup/DOC

 - kw T => trans
 - geom only keeps one trans
ENH: add MriHeadCoreg Traits GUI
ENH: MriHeadFitter

 - assembe list of files to copy on __init__
 - look for source spaces with different iso parameters
STY:  cleanup method order
ENH: MriHeadCoreg gui

 - If target files exist, ask whether to overwrite
 - remove save-trans button
RF: use numpy array instead of matrix
OPT transforms: precompute trigonometric functions