made _issmall() a static member of SymbolicOperator addressing #702 (#703)
made _issmall() a static member of SymbolicOperator addressing #702 (#703)

Co-authored-by: Nicholas Rubin <2c29964c88efb75bf37e3d600851a569240b455f@gmail.com>
kevinsung/OpenFermion,quantumlib/OpenFermion,quantumlib/OpenFermion,quantumlib/OpenFermion,kevinsung/OpenFermion,kevinsung/OpenFermion