Demonstrate switching engines at same and different scales

[x] https://github.com/simularium/vivarium-models/issues/4
[x] https://github.com/simularium/simularium-models-util/issues/16
[ ] https://github.com/simularium/vivarium-models/issues/3

Ideally we would also stitch the segments from ReaDDy back into the filaments before sending them to Cytosim/MEDYAN, but that is beyond the scope of this demo.

simularium / simularium-planning

Demonstrate switching engines at same and different scales #50