limit pvwatts_ac results to greater than equal to 0

[wholmgren]

• Closes pvwatts_ac returns values less than 0 #541
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

I confirmed that the new test fails without the fix.

[wholmgren]

Test failure is unrelated.

[wholmgren]

Hearing no objections, I think we should merge... @cwhanse?