An n-qubit implementation of the quantum fourier transform
Builds heavily off the Qiskit online textbook (alpha) code, found at https://qiskit.org/textbook/ch-algorithms/quantum-fourier-transform.html. My main contribution is building a general state-initialiser based on an input 'state_number' - a denary representation of the binary starting values of the qubits, e.g. 101 -> 5.