Issue_156 Add navigation methods

Added: getParent, getChildren, getInEdges, getOutEdges, getSource, getTarget

Fixed Issue_156

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #157 from GordonSmith/issue_156

Issue_156 Add navigation methods

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>