Add requirement for non-plotting examples to be documented as ipython notebooks

[olgabot]

Somewhat addresses the need for more examples as described here: #262 but not the visual examples, rather the day-to-day grind of dealing with dataframes and creating more dataframes.

• Is it mergable?
• Did it pass the tests?
• If it introduces new functionality in scripts/ is it tested?
  Check for code coverage. To run code coverage on only the file you changed,
  for example flotilla/compute/splicing.py, use this command:
  py.test --cov flotilla/compute/splicing.py --cov-report term-missing flotilla/test/compute/test_splicing.py
  which will show you which lines aren't covered by the tests.
• Do the new functions have descriptive
  numpydoc
  style docstrings?
• If it adds a new plot, is it documented in the gallery?
• Is it well formatted? Look at make pep8 and make lint output
• Is it documented in the doc/releases/?
• Was a spellchecker run on the source code and documentation after
  changes were made?