Convert set-valued functors to acsets

[epatters]

Conjunctive data migration returns a set-valued FinDomFunctor whose sets are not consecutive integers but data tables. We need a procedure to convert these to our standard acset types based on the skeleton of FinSet. The conversion will have non-negligible cost because indices mapping table rows to integers will have to be constructed.

This issue is a complement to #539 with a more straightforward path to implementation.