Resolve "versioning of graph representation"

Closes #30 (closed)

A new library reading an old graph will try to update the graph. When updating is not possible it will raise an exception with a message like this:

Graph version "1.0" requires another library version: python -m pip install "ewokscore>=0.1,<3.2"

An old library reading a new graph will raise an exception with a message like this:

Graph version "2.5" is either invalid or requires a newer library version: python -m pip install --upgrade ewokscore