Statistics widget: ignore skeletons with one node
Statistics widget: ignore skeletons with one node

Do not include skeletons with only one node when counting nodes for the
statistics widget. Closes acardona/CATMAID#198.

fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID