Adding a cumulative energy check functions

[qnguyen345]

• Closes Adding a cumulative energy check function #165
• 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.

I created an energy.py script in the quality folder to check and correct cumulative data streams. This is the github link to the original issue and discussion https://github.com/pvlib/pvanalytics/issues/165

[kperrynrel]

@qnguyen345 can we rename the 10004 file to system_10004_ac_energy.csv? This will make this a little more in-line with the naming conventions in the data folder.

[kperrynrel]

Good first start! Made a couple of nitpicks in the docstrings and suggested updates to the main conversion function. Also we discussed adding an example to the example gallery.