Support memoization for mutual information.

[Schaechtle]

No memo argument is currently accepted for spn.mutual_information. This can be fixed by changing spn.mutual_information to:

    def mutual_information(self, A, B, memo=None):
        # p11 = self.logprob(A & B)
        # p10 = self.logprob(A & ~B)
        # p01 = self.logprob(~A & B)
        # p00 = self.logprob(~A & ~B)
        lpA1 = self.logprob(A, memo=memo)
        lpB1 = self.logprob(B, memo=memo)
        lpA0 = logdiffexp(0, lpA1)
        lpB0 = logdiffexp(0, lpB1)
        lp00 = self.logprob(~A & ~B, memo=memo)
        lp01 = self.logprob(~A & B, memo=memo)
        lp10 = self.logprob(A & ~B, memo=memo)
        lp11 = self.logprob(A & B, memo=memo)
        m00 = exp(lp00) * (lp00 - (lpA0 + lpB0)) if not isinf_neg(lp00) else 0
        m01 = exp(lp01) * (lp01 - (lpA0 + lpB1)) if not isinf_neg(lp01) else 0
        m10 = exp(lp10) * (lp10 - (lpA1 + lpB0)) if not isinf_neg(lp10) else 0
        m11 = exp(lp11) * (lp11 - (lpA1 + lpB1)) if not isinf_neg(lp11) else 0
        return m00 + m01 + m10 + m11