code: public double getGroundHeight(Point2D point, CoordinateSystem3D system) {
  		return getGroundHeight(point.getX(), point.getY(), system);
  	}
docs: Replies the height of the projection on the triangle that is representing a ground.
  <p>
  Assuming that the triangle is representing a face of a terrain/ground,
  this function compute the height of the ground just below the given position.
  The input of this function is the coordinate of a point on the horizontal plane.
  
  @param point is the point to project on the triangle.
  @param system is the coordinate system to use for determining the up coordinate.
  @return the height of the face at the given coordinate.