Bug fixes in connectivity.h (#285)

- Bug fix in tree().
- Rename simpleDigraph() to simpleGraph() (it works for both
directed and undirected graphs).
- Possibly faster implementation for parallelFree() and
simpleGraph().

 SET(PROJECT_NAME "@PACKAGE_NAME@")

 SET(PROJECT_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")