Added sanity check to MinimumSpanningTree() to ensure that the number of edges...

Added sanity check to MinimumSpanningTree() to ensure that the number of edges in the MST is between 1 and 2 times the number of nodes on the island, unless there is only one node, and thus no edges.  Also changed QList to QSet in RandomBFS() as the structure is primarily used to check graph node membership, which is nearly constant with a set but a binary search with a list, and the preserved order is not used.  Implemented a complete test of MinimumSpanningTree(), and by extension, GetNodeConnections() and RandomBFS().

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4263 41f8697f-d340-4b68-9986-7bafba869bb8