BF+TST: make sure labels stay in order for exclude

Previously passing ``exclude`` to the ``parcels`` generator would make
the order of labels undefined, because we use ``set(labels)``.  Keep the
order of labels; test that we do.
NF: add MNI basal ganglia definitions for example

Process MNI template atlas a little to get basal ganglia definitions.
Make README with MNI data license.
NF: add example for parcels generator

Small example to show extracting mask for unique ROI values in an image.