CLN: move generic code to NDFrame

[jreback]

thinking of:

lots of parts of align
comparison ops,
parts of apply (and put special cases in the sub-classes)
pop
align & where

[paulproteus]

I notice that the 'def pop()' in pandas/core/frame.py simply calls out to NDFrame.pop(self, item). It seems to me that the implementation will automatically call out to NDFrame since DataFrame is defined as a subclass of NDFrame.

A question for pandas core maintainers: is the right fix for pop() to just delete it from DataFrame?

For the other methods, more creativity may be required.

For anyone interested in addressing this issue: The great thing about pandas having a test suite is that you can willy-nilly delete, or move, code and see if it breaks pandas by just running the test suite!

[jreback]

closing in favor of PR #2915