[ENH] multi-source dijkstra

[jGaboardi]

Implement a multi-source dijkstra algorithm. Following this enhancement, the generation of network voronoi diagrams will be possible.

pre-reqs:

• new functionality for creating nodes along network arcs (something like insert_node)
• etc.

See:

• Atsuyki Okabe and Kokichi Sugihara. Spatial Analysis Along Networks. John Wiley & Sons, Inc., West Sussex, UK, 2012. doi:10.1002/9781119967101.

[jGaboardi]

#380