Generates two-body positivity constraints on fermionic RDMs.

The specific constraints implemented are known positivity constraints
on the two-fermion reduced density matrices. Constraints are generated
in the form of FermionOperators whose expectation value is known to be
zero for any N-Representable state. Generators are used for efficiency.

n_orbitals(int): number of spin-orbitals on which operators act. n_fermions(int): number of fermions in the system.

Constraint is a FermionOperator with zero expectation value.