Add iterators

[joloppo]

• makes validate(), pt.DataFrame.validate() & pt.Model.validate() return the dataframe being validated
• adds filter_columns: bool option to all the validate()s
• adds iter_models() to pt.DataFrame which returns an iterator of patito model instances
• adds to_list() to pt.DataFrame which returns a list of patito model instances
• adds iter() to pt.Model which returns an iterator of patito model instances

• adds to_list() to pt.Model which returns a list of patito model instances

  Questions

  • happy to remove the ListableIterator
  • should I add an example to the readme?
  • how to update docs (by the way I don't think the website docs are up-to-date)

[thomasaarholt]

Hi @joloppo! First and foremost, thank you for an excellent PR! Jakob and I really appreciated the work you put into this, it's clearly very thought-through!

We've accepted most of your additions. I suggest you take a look at the commits which have explanations for our modifications. I'm going to merge this right now, and we can follow up with anything else in an additional PR.

Thank you for your work!

[joloppo]

Awesome, thanks guys!