Move graph prep to GraphCreator classes.
Move graph creators to their own package.
Delete GraphLoaderUtilities.

This has been replaced by GraphCreator.
Rename and move GraphCentralityUtilities ->
ClosenessCentralityUtilities.
Refactor abstract executor functionality to helper

Most of the work done in GraphAnalyzer could be pushed up to a
superclass called AbstractExecutorFunctionHelper which will be reused by
other SQL functions extending AbstractExecutorFunction.