Add medians a feature

[j-t-t]

Open street maps (and Boston's data before that) has roads with medians consist of two separate roads. We want to identify medians and add it as a feature, but keep the representation otherwise the same.

This also requires fixing the case where there is a t-intersection on one side of the median but no intersection on the other. Currently, the stretch on the other side of the median is part of the intersection, but we don't want that to be the case.