Restructured KDE module to make it easier to add more kernels and network methods.
Changed imports where required.
Added Toby's ITN class and network KDE class.
Created blank module (TODO) that will contain the network-based SEPP implementation.
Modified OSMRenderer to allow buffering lines - makes them appear nicer in the plot, and allows better control of width.  However, failed to add edgecolor as the multiple line segments overlapping look ugly.  TODO?