Improved support for left pushforward data migration

Thanks to @slibkind's PR #433, we now have a first implementation of the left pushforward data migration functor for C-sets. Given a functor F: C -> D between schemas, the algorithm assumes that the graph generating D has no cycles (it performs a topological sort on D). This means that we cannot, for example, create the free reflexive graph on a graph because the schema for reflexive graphs has cycles.

In general, computing left Kan extensions is a challenging problem. The state of the art seems to be [Fast Left Kan Extensions using the Chase](https://www.categoricaldata.net/cql/fmc.pdf) by Spivak and Wisnesky. For a next step, I think we should just do the simplest thing that would cover standard examples like symmetric and/or reflexive graphs.