python-graphviz has changed output variables in Ubuntu 16.04

[Jon Hill]

data_overlap needs a fix to account for change in GraphViz output

On LTS - data overlap works fine.

On 16.04 you get:
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/stk_gui/interface.py", line
1150, in run_data_overlap
    f.write(str(i)+","+key+"\n")
TypeError: cannot concatenate 'str' and 'set' objects

So GraphViz is returning a set now, not a dict.