v0.3.0: 2020 Q1 PUDL release in support of data archiving

This release is mostly about getting the infrastructure in place to do
regular data releases via Zenodo, and updating ETL with 2018 data.

Added lots of data validation / quality assurance test cases in
anticipation of archiving data. See the pudl.validate module for more
details.

New data since v0.2.0 of PUDL:
* EIA Form 860 for 2018
* EIA Form 923 for 2018
* FERC Form 1 for 1994-2003 and 2018 (select tables)

We removed the FERC Form 1 accumulated depreciation table from PUDL
because it requires detailed row-mapping in order to be accurate across
all the years. It and many other FERC tables will be integrated soon,
using new row-mapping methods.

Lots of new plants and utilities integrated into the PUDL ID mapping
process, for the earlier years (1994-2003).  All years of FERC 1 data
should be integrated for all future ferc1 tables.

Command line interfaces of some of the ETL scripts have changed, see
their help messages for details.