`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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Notice: Issues with the "new contributor" label are issues assigned to a specific individual who may either be a new contributor to this repository or who have only have had minimal prior experience. These issues refer to a specific pre-arranged set of action items, and are intended to welcome and onboard new developers. Please get in touch with mkhorton@lbl.gov if you would like to get involved and want a similar "new contributor" issue assigned.

[hmlli]

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