REF: move dispatch methods to WrappedCythonOp

[jbrockmendel]

This gets us most of the way towards having everything in the correct class.

The sticking point that makes this not-a-slam-dunk is that a check is removed:

            if self._filter_empty_groups and not counts.all():
                assert result.ndim != 2
                result = result[counts > 0]

This condition never holds in the tests, and the assertion would fail if it ever did. I don't have a compelling reason to remove this check other than it being inconvenient to move it to WrappedCythonOp.

[jreback]

can we move WrappedCythonOps the class to a new module, maybe cython_ops.py? (can do it in this PR or pre-cursor)

[jbrockmendel]

at some point. naming-wise i actually think it makes sense for it to be in ops.py and everything else to be somewhere else, but im not sure where that would be (the relationship between Grouper vs BaseGrouper still confuses me)

[mzeitlin11]

Could we make _ea_wrap_cython_operation and some of the other private methods called from an instance of WrappedCythonOp public? Are there are any rules of thumb we have for private/public methods for non-user facing classes?

[jbrockmendel]

good catch. i think before long _cython_operation itself will move, at which point it will become public

[jreback]

lgtm can you merge master

[jbrockmendel]

Rebased + green