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.

OperatorSpecificationError Operator is not two-body number conserving.