Adds ability for soiling.Hsu to accept arbitrary time intervals (other than 1 hour)

[jsstein]

• Closes pvlib.soiling.hsu model implementation errors #970
• I am familiar with the contributing guidelines
• [no changes needed] Tests added
• [N/A] Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/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`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

[kandersolar]

Some quick comments. Would also be good to add a test that uses non-hourly data.

[wholmgren]

@cwhanse @kanderso-nrel @jsstein what's needed to move this PR forward? Looks pretty close to me.

[kandersolar]

Other than the small whatsnew changes, the only thing I see is that the test has @needs_pandas_0_22, which is no longer necessary as of #1003

[kandersolar]

FYI test failure is the shapely segfault

[wholmgren]

a couple of suggestions/questions but ok to merge

[wholmgren]

Thanks @jsstein and @kanderso-nrel!