Graph Parition and Modularity Updates

Changes to GraphPartition behavior, and Newman Spectral Partitioning

1. GraphPartition class will now raise an error if a graph with a weighted adjacency matrix is passed
2. find_solitary GraphPartition method added to check for nodes with degree 0
3. Update to some doc strings
4. Newman Spectral Partitioning: changed delta modularity calculation, which should reduce overly optomistic estimates of partition impact on full modularity calculation, this requires further analysis for optimal solution