Guard rails are currently added to OSI as stationary objects. The problem is, that this way, they cannot have a height profile, so the guard rails will not follow the elevation of the road. A better way is to implement guard rails in OSI as lane boundaries.

There are some open points, that should be discussed:

• Would it be possible to distinguish between a barrier and a guard rail, as these are separate types in OSI.
• Currently the outline of a guard rail always goes to one side and then comes back, so I am taking only half the corner points. Is this generally correct?
• The boundary points do not store the width of the barrier or guard rail. So in this PR, the width is set to a default value of 0.1 m. This would be fine for me, but maybe there is a smarter way to set a width.

Fixes #758