Cleaned-up docstrings in json_parser.
Added note on growing binary trees while iteration is ongoing.
Added the avoid modification note to DFSITerator of graphs.

Also revised the wording of the the note in InorderIterator.
Modified the base class of graphs.

Added the properties added_nodes and edge_count to the base class.
Issue: Some classes are working for Python2, some for Python3. Noted.
Framework for implementing DFSIterator.
Finished but surely flawed implementation of DFS iterator. Untested.