CLN/ENH: Make names getattr'able in core/ops +PEP8.

Now the name passed to a method can be used to get that method from the
NDFrame (previously you had things like `or_ [|]`. better to just have
them all be named exactly what they are s.t. subclasses can do
`getattr(super(self, MyClass), name)(*args, **kwargs)`.
Merge pull request #5240 from jtratner/cleanup-core_ops

CLN/ENH: Make names getattr'able in core/ops +PEP8.