Detect that classes which extend moodleform* call parent:: in overridden functions

[jrchamp]

We ran into a few issues when parent:: wasn't being called for an overridden function, so I was inspired by #106. I thought maybe this would go into #92, but it should probably be applied to core code also. Here's a quick list of examples that would be good to include sooner rather than later. I hope this helps! ❤️

Call parent::

• moodleform
  • set_data
  • validation
• moodleform_mod
  • data_postprocessing
  • data_preprocessing
  • definition_after_data
  • standard_grading_coursemodule_elements
  • (and all from moodleform)

Method override should not exist

• moodleform_mod
  • get_data