Create needed infrastructure in Graph to enable writer classes

[davidalber]

Geneagrapher currently generates only DOT output and the generation of the dot content is done within the Graph class. We want to introduce "writer" classes to Geneagrapher that will generalize this conversion and make it more flexible. Writing the output will be done outside of the Graph class.

A writer class takes a Graph object when initialized and uses the Graph interface to get the information needed for writing the graph.

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Prerequisite of #6.