Permute a product of creation FermionOperators so that the index goes

from the lowest to the highest inside of each spin sector and multiply by the correct parity.

ops (FermionOperator) - A product of FermionOperators

coeff (complex64) - The coefficient in front of the operator strings

order (string) - specify if the operators should be returned in numerical order or not

(FermionOperator) - A product of FermionOperators with the correct sign