Update the Path concept
Concept check for paths

DirPath renamed to Path
The interface updated to the new lemon interface
Make difference between the empty path and the path from one node
Builder interface have not been changed
// I wanted but there was not accordance about it

UPath is removed
It was a buggy implementation, it could not iterate on the
nodes in the right order
Right way to use undirected paths => path of edges in undirected graphs

The tests have been modified to the current implementation

 /// The namespace of LEMON

 /// The namespace of LEMON

 ///

 namespace lemon {

   /// The namespace of LEMON concepts and concept checking classes

   /// The namespace of LEMON concepts and concept checking classes

   ///

   namespace concept {}

 }