Remove networking code

The decision was made to move the network code out of GeoTrellis core and move it to GeoTrellis Transit (the geotrellis-transit repository on the geotrellis github account). The shortest-path logic and graph datastructures are not in line with the main focus of GeoTrellis, although using GeoTrellis to convert those structures into Rasters is. So GeoTrellis Transit uses GeoTrellis core to work with those graph data types and logic.

This PR also includes making an implicit class that lets ```.collectSources``` to be called on a sequence of sources, which has a similar effect to the ```logic.Collect``` operation.