updates for Python 3

[nsheils]

In order to make this work for python 3, instances of "import cPickle as pickle" need to be changed to "import pickle"

Additionally, in data_utils.py the load_graph function must be changed from "node_maps.iteritems()" to "node_maps.items()" . Similar in graph.py

Further, all print statements now need parentheses.

[nsheils]

aggregators.py, encoders.py and train_helpers also need instances of .iteritems() changed to .items()

An additional function that needs changing is "xrange" to "range" in train_helpers.py line 48, model.py line 49, and utils.py line 51, 54, 57, 58