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.

#cmakedefine HAVE_LONG_LONG 1

#cmakedefine HAVE_LP 1

#cmakedefine HAVE_MIP 1

#cmakedefine HAVE_GLPK 1