Puts a two-body fermion operator in chemist ordering.
The normal ordering convention for chemists is different.
Rather than ordering the two-body term as physicists do, as
a^\dagger a^\dagger a a
the chemist ordering of the two-body term is
a^\dagger a a^\dagger a
fermion_operator (FermionOperator): a fermion operator guarenteed to
have number conserving one- and two-body fermion terms only.
chemist_ordered_operator (FermionOperator): the input operator
ordered in the chemistry convention.
Operator is not two-body number conserving.