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