Support dict-like database migration

Objects like HDF5 files, SQL databases, Mongo Databases, and dicts of chunks of things might all be convertible. One could also imagine adding a very thin `Directory` type that expressed a filesystem hierarchy as a nested record type.

A natural common interchange format here might include dicts of either `chunks(DataFrame)` or `chunks(ndarray)` depending on the underlying dshape. This would also be a good place for `chunks(dynd.nd.array)`.

The issue names no files, tests, or entry points and proposes several possible interchange forms. Start by reading the repository’s existing conversion and migration paths; done would require a decided dict-like representation and agreed support for the listed sources, with tests documenting that behavior.

Written by the indexing model from the issue text.