make imports explicit in __init__.py (#612)
make imports explicit in __init__.py (#612)

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