Data migration with linear queries and gluing queries

[epatters]

This issue is the dual of #535: we want data migrations from C-sets to D-sets specified by a functor D -> Diag(C), where Diag(C) is the (covariant) category of diagrams and their morphisms. I am calling these "gluing queries" or "agglomerative queries" because they involve computing colimits.

An important special case is when the diagrams are discrete, meaning that coproducts are computed. David Spivak calls these "linear queries."