EPIC: Support digraph6 file format

[john-boyer-phd]

Nauty supports output of a "digraph6" or ".d6" file format for digraphs. Implement a reader and writer for this format.

Include the ability to flag whether to ignore or to report an error and fail for multiple edges and self-loop edges (or to allow them, if support is added for either of them).

When transforming a graph into the "digraph6" format, the letter 'd' is used, i.e., planarity -t [-q] C|-td I O.