enhancement - added support for volumetric parcellations from freesurfer

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@7708 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0
Merge branch 'master' of github.com:fieldtrip/fieldtrip