Return self.flattenedPages as iterator instead of list

[ninnzz]

I've noticed that the self. flattenedPages is returned as a list.
For several purposes, I think returning it as a iterator is better than just a pure list.

• More intuitive in accessing the pdf results

pages = pdf.flattenedPages
next(pages)

This is just a suggestion. What do you think?

[MartinThoma]

@ninnzz I don't see any place where we iterate over flattened_pages (it was renamed). How would this improve things?