FIX: Clearer warning for discrete source spaces
ENH transforms: tools for coregistration and scaling MRIs
FIX circular import 
ENH:  use ConfigParser to store scaling info
ENH coreg:  single function for label scaling
TEST coreg:  include source space;  scale with 3 parameters
FIX TEST scale_mri
FIX:  requires_mne_fs_in_env testing decorator
STY:  rename s_to, s_from -> subject_to, subject_from
API:  privatize decimate_points and is_mri_subject
ENH:  is_mri_subject
DOC
API:  expose public functions
API apply_trans():  trans has to be 4x4
OPT fit_point_cloud:  use scipy's KDTree
API:  rename fit_matched_pts -> fit_matched_points
OPT apply_trans:  avoid memory re-allocation
FIX als_ras_trans:  make it 4x4
OPT fit_point_cloud:  use scikits.learn BallTree
DOC