Updates/consistency fixes for opening section
Fixed Kruskal's example, converted psuedocode description into algorithm
format, added helpful function to problem 1, expanded networkx description
and fixed problem 2, general edits for consistency and clarity
New solutions file and edited image segmentation section; problems 3 and 4
still need editing (mostly hints) and the solutions file is incomplete
Merge pull request #703 from marissa-graham/image_seg

Updates for minimal spanning trees.