Return the indices for n_electrons in n_qubits under JW encoding

Calculates the indices for all possible arrangements of n-electrons within n-qubit orbitals when a Jordan-Wigner encoding is used. Useful for restricting generic operators or vectors to a particular particle number space when desired

n_electrons(int): Number of particles to restrict the operator to n_qubits(int): Number of qubits defining the total state

indices(list): List of indices in a 2^n length array that indicate the indices of constant particle number within n_qubits in a Jordan-Wigner encoding.