MigrationGraph component

[hmlli]

Adding a component that deals with ion migration event visualizations. Works with MigrationGraph object from pymatgen-analysis-diffusion.

[mkhorton]

Thanks @hmlli! I added a "new-contributor" label to this issue for organizational purposes.

This PR will likely involve:

• Addition of a new file, crystal_toolkit.renderables.migration_graph
• Possibly the addition of a new MPComponent, to crystal_toolkit.components.migration_graph (depending on whether this can use the existing StructureMoleculeComponent or not, perhaps it can subclass)
• A small example app in crystal_toolkit.apps.examples so that this can be added to the test suite and documentation

If you have any questions related to the above, please either ask them in this issue or contact privately.

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[hmlli]

A WIP PR has been added to address this issue: #267