added test case to CompositeDataUnitFactory to check for null parameters
in the makeFromDataUnitFactories method and throw a NPE.
also implemented the functionality in the CompositeDataUnitFactory
corrected typo (package pdusent -> pdusend)
added FunctionDataUnitFactory
added sizeInBits method in FunctionDataUnit and the sizeInBits field to
specify the size of a FunctionDataUnit