FIX: make average robust to missing (padded) data


git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@5307 42a5c34f-2066-0410-bec5-ba365beb4995
DOC


git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@5308 42a5c34f-2066-0410-bec5-ba365beb4995