add pnnl data parser

[wholmgren]

• [x] Closes #563 .
• [x] I am familiar with the contributing guidelines.
• [x] Tests added.
• [x] Updates entries to docs/source/api.rst for API changes.
• [x] Adds descriptions to appropriate "what's new" file in docs/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

[wholmgren]

Works but 1 year of data exceeds the API limit.

2021-04-16 14:13:49,472 INFO Updating PNNL ghi from 2017-01-01 00:00:00+00:00 to 2018-01-01 00:00:00+00:00.
2021-04-16 14:13:49,611 DEBUG Posting data to PNNL ghi between 2016-12-31 16:00:00-08:00 and 2017-12-31 15:59:00-08:00.
2021-04-16 14:13:53,701 DEBUG Resetting dropped connection: dev-api.solarforecastarbiter.org
2021-04-16 14:13:56,087 DEBUG https://dev-api.solarforecastarbiter.org:443 "POST /observations/8abae936-9ef7-11eb-881b-0a580a8003fa/values HTTP/1.1" 413 80
2021-04-16 14:13:56,087 ERROR Posting data to PNNL ghi failed.
2021-04-16 14:13:56,087 DEBUG HTTP Error: {"errors":{"413":"Payload Too Large. Maximum payload size is 16777216 bytes."}}

[wholmgren]

3 months works

solararbiter referencedata update --network 'PNNL' --base-url 'https://dev-api.solarforecastarbiter.org' --start 20170101 --end 2017040