Deprecate DataobjImage.get_data in nibabel 3.0

Per the changelog, `DataobjImage.get_data()` was scheduled for deprecation in April 2018, but we missed that. Tools that are stuck on Python 2 aren't likely to get updated, so by leaving this change out of the 2.5 series, we can give them a bit of a soft landing.

I'm proposing that we officially deprecate in 3.0, remove in 5.0. Please make noise if you have objections of any kind.