ENH: allow to unmask multiple "volumes" in one call

logically multiple samples should have gone into rows, but to mimiq
nibabel order (x,y,z,t) with t in the last dimension and stay coherent
with order of dimensions as returned by analyze_features -- multiple
volumes are assumed to reside in colums
Merge pull request #18 from yarikoptic/master

ENH: allow to unmask multiple "volumes" in one call