Fix wrong semantics in add_lanelink()

[johschmitz]

The documentation of the add_lanelink() method for connecting roads was slightly wrong, confusing the "outgoing" with the connecting road.

[mander76]

I prefer not to change variable names if not really necessary (breaks backwards compability), feel free to update the docstring though.

[johschmitz]

I guess later it will be even more difficult to change this but okay I can do it only for the doc string. This will still result in problematic code though if someone explicitly set the argument. Maybe in this case it would be better to let the code fail and then people will see that they have to rename the argument which is quite an easy change. I would say semantical clean ups like this are kind of okayish in a 0.x phase of a project. If OpenDRIVE 1.9 or 1.10 does start to actually define outgoing roads/lanes this will really be a problem since then the argument name kind of really becomes wrong so I would maybe fix it as long as it is still possible but it is your call. Will change it when back from holidays if you don't change your mind. Otherwise I could for sure fix all failing tests.