double OETanimoto(const OEBitVector &a, const OEBitVector &b)

Calculates the Tanimoto similarity coefficient between two OEBitVectors. This is defined as the number of bits that the two bitmaps/fingerprints have in common divided by the total of bits set in either. This function does not require that the two OEBitVectors be the same size, the bits lacking from the shorter OEBitVector being interpreted as all zeros. This function always returns 1.0, if the two OEBitVectors are equal, even if they are both empty.