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.