GraphvizDispatcher

[ericmehl]

This adds a new dispatcher for debugging and understanding task batches. GraphvizDispatcher writes out a file in DOT format (https://graphviz.org/doc/info/lang.html) that can then be visualized in many ways using Graphviz tools. Frame numbers can optionally be added as well as context variables based on wildcard matches.

Since this is more of a developer tool, I'm including it in GafferDispatchTest. This dispatcher can be created one-off using the Python editor, or included in a menu using startup configs.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.