Handle -99999 NaN for 6-character width fields in USCRN data

[lboeman]

• [x] Closes #773 .
• [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.

[lboeman]

Re-imported data from April 2021 until now and recomputed the Reference forecast for Millbrook, NY here:"https://dashboard.solarforecastarbiter.org/forecasts/cdf/7eeb7264-6f26-11ec-a683-0a580a800248?start=2021-12-20T17%3A42%3A00.000Z&end=2022-01-11T04%3A00%3A00.000Z