Create a simple generic GraphViz renderer for DiGraph (#1034)

* Create a simple generic graphviz renderer for DiGraph
There are three basic kinds
- A simple default renderer
- A ranked renderer that places nodes in columns based on depth from sources
- A sub-graph render for graphs that contain a loop
  - Renders just nodes that are part of first loop found
  - Plus the neighbors of the loop
  - Loop edges are shown in red.

* Create a simple generic graphviz renderer for DiGraph
- Moved the graph loop finder into DiGraph
- Fixed scala doc per Edward's comments

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