Modify the interface of MinCostArborescence + improvements (#267)

- Rename arborescenceValue() to arborescenceCost().
- Rename DefXyz template named paramaters to SetXyz.
- Rearrange public functions (for better doc).
- Doc improvements.
- Extend the test file with interface checking.

 SET(PROJECT_NAME "@PACKAGE_NAME@")

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