Added missing to_python converter for ConstWeakLanelet

[brandesjj]

First of all: thanks for the great work!
I was playing around with the Python API and loading RightOfWay Objects from the sample map.
However, access to the 'right_of_way' and 'yield' keys of the parameters attribute of the RightOfWay Object was not possible due to a missing to_python converter for ConstWeakLanelet.
I'm not sure whether this was intentional or not.

However - I added it, direct access through the parameters attribute now works and shows the same results as calling the rightOfWayLanelets() method. Same goes for the 'yield' key and yieldLanelets() method.

This is related to #233

[immel-f]

Thanks for the fix! This was likely an oversight 👍