Add "schematic view" plotting function

[patmalcolm91]

Add a plotting method to the Net class which plots junctions as points and edges (or lanes?) as lines in data coordinates. This would allow viewing large networks from a very zoomed-out view. Things to consider:

• option to draw edges/lanes as straight lines (ignore actual geometry)
• edges/lanes should be (optionally) offset to the right to avoid overlap on two-way streets
• should provide flexible and easy-to-use control over color and other style parameters