Added new data files for testing

cgd112p020-AK00.json contains all the district polygons except the main
land mass of Alaska.  That polygon has over 70,000 coordinates, so it
caused a memory error.  I rewrote the section of code that couldn't
handle that many data points and ran Alaska separately.
Changed the data structure used for a spanning tree.

The spanning tree used to be represented by a very sparsely populated 2D
array of integers.  This was taking up way too much memory to process
polygons with lots of coordinates (looking at you, Alaska), so I changed
the representation to a more compact list of lists of connections.