• Closes #xxx
• Added tests to cover all new or modified code.
• Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings.
• Added new API functions to docs/api.rst.
• Non-API functions clearly documented with docstrings or comments as necessary.
• Adds description and name entries in the appropriate "what's new" file
  in docs/whatsnew
  for all changes. Includes link to the GitHub Issue with :issue:`num`
  or this Pull Request with :pull:`num`. Includes contributor name
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• Pull request is nearly complete and ready for detailed review.
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gaps.completeness and gaps.completeness_score require input to be a Series with DatetimeIndex, but also optionally accept a freq parameter for the input Series. If provided, the freq parameter is used to compute the number of seconds between Series values. The path for calculation involves pandas functions (infer_freq and to_timedelta) that are inconsistent in their use of frequency strings. See this error for the possible result.

pandas has a reported bug pandas-dev/pandas#36769 for several years and it appears unlikely that the behavior will be changed.

The number of seconds can be had from the Series index, so I'm removing the freq parameter as a way to avoid the pandas issues.