DataflowIterator normalization

The patch normalizes the data flow iterators. The reasoning behind it is
to allow passing the base class around without knowing what underlying iterator
is used. This will thus allow called functions to call the
specified Next function. This feature will be required for future patches.

- Made DataflowIterator a base class with an abstract Next function
- Updated each derived class to use the same Next function signature
- Added comments and doxygen comments

Change-Id: I3b9bce6326675575172f0ebd3681369d40d55661
Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>