Added converters for DDOT and HiDeF

• Add HiDeFToHierarchyConverter, a class to convert a edge list and node list
  in HiDeF format to hierarchy in HCX.
• Add InteractomeToDDOTConverter and DDOTToInteractomeConverter, classes to convert network in
  CX2 format to DDOT format and vice versa, HierarchyToDDOTConverter and DDOTToHierarchyConverter,
  classes to convert hierarchy network in HCX format to DDOT and vice versa.