pep8/pylint cleanup on symmath/formula.py

@carsongee this file has an absurd amount of pep8 & pylint violations. 

I cleaned up the pep8 and took a stab at some Pylint. Would you mind looking these over?

If you have a spare 15 minutes, could you add a commit that further cleans up Pylint by adding docstrings to functions? I don't know this code well but it's used by MIT so I'm hoping you can help.