simple DRC: check for multiple drivers
fix source of PortTypeValues used for comparison
fix position of comments in test case
Merge branch 'master' into issue119-simple-drc

Conflicts:
	src/main/java/org/manifold/compiler/back/digital/Netlist.java
fix null pointer bug on certain test runs
fix nits, remove netlist port/connection type getters
(the Schematic is always the place to go for those)
Merge pull request #130 from mtrberzi/issue119-simple-drc

Design Rule Check: no multiple drivers