v0.2.0: Data package based output without PostgreSQL

This is the first release of PUDL to generate data packages as the
canonical output, rather than loading data into a local PostgreSQL
database. The data packages can then be used to generate a local SQLite
database, without relying on any software being installed outside of
the Python requirements specified for the catalyst.coop package.

This change will enable easier installation of PUDL, as well as
archiving and bulk distribution of the data products in a platform
independent format.