Made some functions const.

  * Taken into account the direction of the road segment.
  * Added a margin at the beginning and the end of the new road.

Modified the generate functions of the spline to create a suingle point spline when the length is 0 or the end point is further than the start point (invalid spline).
Modified the get_closest_point function to fail when the gradient goes beyond limits.

The spline class returns maximum length when the is no closest point belonging to the spline.
All other classes check the returned length to check for the existance of the closest point.

* to get the pose at a given distance in the link.
* to get the curvature at a given distance in the link.
* to get the closest node, node index, lane, lane index, segment, segment index and pose to a given point.
* to get the number of lane objects and a reference to them.
* to udpate the index of lanes and nodes when creating new roads.
* to update the node index from a path.

* to get the pose at a given distance in the link.
* to get the curvature at a given distance in the link.
* to get the closest node, node index, lane, lane index and pose to a given point.
* to get the next segment in the road.
Added the link reference update to the clone() and get_sub_segment() functions.

* to get the pose at a given distance in the link.
* to get the curvature at a given distance in the link.
* to get the closest node, node index and pose to a given point.
Added the link reference update to the clone(),get_sub_lane(), update_start_node() and update_end_node() functions.

The get_closest*() functions have a new parameter to limit the search to the links belonging to a lane.
The parent information no longer includes the length of the link.

* to get the pose at a given distance in the link.
* to get the curvature at a given distance in the link.
The length is no longer used to generate the spline. It is internally computed.
The curvature is provided when updating the start or end positions.

The links keep the splines to generate the trajectory.
Solved a bug with the computation of the curvature for lanes other than the central one.

Improved Exception report information.
clean_references() functions no longer update references.

Added a function to add additional node in unconnected lanes.
Solved some bugs.

Added functions to update all the references when a new road is created.

Added functions to create a copy of the segment and update its references.
Added a function to clean any invalid reference.

Added functions to create a copy of the lane and update its references.
Added a function to clean any invalid reference.
Added a function to get the closest point on the lane to a given point.

Added functions to create a copy of the node and update its references.
Added a function to clean any invalid reference.

Added the forward declaration of all classes.

Added functions to update the start and end positions of the link.
Used a single update_reference() function with all the new segment,lane and node references.

Added the path to the Find cmake file and CMakeLists.txt files.

  * A function to get the closes node to a point.
  * A function to get the set of closest points to a point.

  * A function to get the link ending at a given node.
  * A function to get the closest link to a point.
  * A function to get the set to closest points to a given point.