Fixed small issue in line_intersector.coordinateIntersection
Updated algorithm/cg_algorithms.dart

To work with LineSegments instead of coordinates (where appropriate)
More implementation of geomgraph.index

Almost all changes were in the geomgraph.index library, which should
be almost complete, needs thorough testing. Will start that tomorrow
EdgeSetIntersectors now return IntersectionInfo objects, rather than
the raw intersection, to allow attaching extra data which is required
by the edges without having to recompute it.

Decided to use the quiver library for access to optional and range,
but still need to factor out the range dependency before
removing it.