Create nighttime_offset_correction function

[AdamRJensen]

• Partly closes Offset calculation and correction for irradiance data #170
• 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
  and/or GitHub username (link with :ghuser:`user`).
• 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.

This PR proposes one method for calculating the nighttime offsets discussed in #170. It is by no means a suggestion for this specific implementation to be the way forward, but making a PR simple makes it more efficient to discuss this particular implementation.

[AdamRJensen]

The current implementation manually corrects midnight to be included in the day before/after corresponding to whether the data is right or left labeled, and then passes the adjusted time series to .groupby:

grouping_category = irradiance.index.date
if label == 'right':
    grouping_category[irradiance.index.time == dt.time(0)] += -dt.timedelta(days=1)
nighttime_offset = nighttime_irradiance.groupby(grouping_category).transform(aggregation_method)

However, this could automatically be handled by the .resample function which natively supports the label parameter:

nighttime_offset = nighttime_irradiance.resample('1d', label=label, closed=label).transform(aggregation_method)

If reviewers agree, I prefer the latter implementation as it's a lot cleaner and easier to read.