enhancement - added (as of yet undocumented) method mean_thresholded for parcellations (only for matricial inputs and averaging across first dim)

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8687 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0