resolve multigraph issue with value unpacking

[thomcsmits]

Addressing issue #13
Currently, when trying to convert a networkx multigraph, a ValueError is thrown. This is caused by to_pandas_edges, where the edge keys are added to the attributes.
This PR simply checks if the graph is a multigraph, and if so, skips this step.

[T-Flet]

I am not sure whether GitHub would have notified you of this, but I opened and accepted an instance of this pull request in my fork (which was originally made precisely to have a version with fixes from issues and PRs).

The fork has now been detached and released as its own library, altair-nx, because the original has been inactive since 2020 and because the fork diverged from it considerably after a full code-base rework and the implementation of new features (e.g. curved edges, self loops, and much greater customisability).

[thomcsmits]

@T-Flet Ah thanks for the @. I indeed had not gotten a notification from GitHub. Great work! I'm glad you've separated it out and continued to improve its flexibility! I'll look more into altair-nx later :)