Correcting changeSource interface and documentation

- The changeSource() and changeTarget() is renamed to changeU() and
changeV() in undirected graphs
- The changeSource(a, n) and changeTarget(a, n) is removed from
undirected graphs
- Correcting invalidating iterators in documentation

 /* Define to 1 if you have CPLEX. */

 #undef HAVE_CPLEX

 /* Define to 1 if you have GLPK. */

 #undef HAVE_GLPK