Fix clang++ warning in AtomPairs.cpp

[greglandrum]

/scratch/RDKit_git/Code/GraphMol/Fingerprints/AtomPairs.cpp:448:78: warning: operator '<<' has lower precedence than '+';
'+' will be evaluated first [-Wshift-op-parentheses]
atomCodes.push_back(((_atomInvariants)[(_atomItI)->getIdx()])<<1 + 1);
~~~~^~~
/scratch/RDKit_git/Code/GraphMol/Fingerprints/AtomPairs.cpp:448:78: note: place parentheses around the '+' expression to
silence this warning
atomCodes.push_back(((*atomInvariants)[(*atomItI)->getIdx()])<<1 + 1);

[greglandrum]

Fixed with 38ca41c