ENH: Increase coverage for defuzzify submodule; remove `vertex` function

This PR substantially raises coverage in the defuzzify submodule, increases the efficiency of `skfuzzy.lambda_cut`, and corrects a couple minor style issues in comments / typos in skfuzzy/defuzzify/defuzzify.py.

It also removes the `vertex` function, which depended on the dangerous function `eval()` which did not behave the same in Python 2 and 3. Retrofitting for use with e.g. SymPy might be possible, but this is a pretty esoteric function and the only use of `eval()`. I'm reluctant to add a SymPy dependency at this time for such a minor reason.