Fix and add test to ensure proper handling of merges by adjust_partition function

1) Adjusted variables after merges in the adjust_partition function in modularity.py  
This was necessary because the mapping of the GraphPartition "index" may no longer match the adjust_partition "node_map" dictionary, due to changes in module numbering after merges.
2) Added check for proper handling of merges to the "test_adjust_partition" function in test_modularity.py.