Added functions to get the left and right lanes.
Added functions to get the left and right road marks.

Merge branch 'master' of ssh://gitlab.iri.upc.edu:2202/mobile_robotics/autonomous_driving/autonomous_driving_tools

Added images needed by the documentation.

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.