Rewrite pandas.util.testing.assert_almost_equal for performance

[wesm]

I don't know if anyone's profiled the test suite, but we'd get major performance boosts if we wrote a Cython version of assert_almost_equal. Is there another open issue for this already?

[cpcloud]

@wesm i think you have one somewhere

[cpcloud]

i can't find it though maybe it was something else

[danbirken]

FYI: #3150 is the duplicate of this one that was theorized to exist in the initial message