Fix fractfact_opt for Python3

[relf]

Resolves #9

[relf]

@jvailius, your example fractfac_opt(4, 1) returns now ('a b c abc', ['a = bcd', 'b = acd', 'c = abd', 'd = abc', 'ab = cd', 'ac = bd', 'ad = bc', 'abcd'], array([0., 0., 3., 4., 0., 0., 0., 0., 0., 0.]))
Does it make sense to you? I can add an assertion on the return value but I am not familiar with the function so just to check it is ok for you.

[jvailius]

Does it make sense to you?

I have not the faintest idea to be honest :D I am relatively new to DOE but this looks like an interesting function and I will further explore it. The result is identical what I get with my fix. Thank you.