Network::graphviz() bugfix

Graphviz output function wasn't handling singleton nodes correctly,
resulting in an endless loop.  That's fixed, and I also simplified some
unnecessarily complicated loops.
Adding simpler way of calling Network::transitivity()
revised/cleaned up test_network example
revised/cleaned up examples Makefile
Merge branch 'master' of github.com:tjhladish/EpiFire

Conflicts:
	src/Network.h