added a convenience function for calculating the spectrum that corresponds to two-time correlation data using fft
added a convenience function for calculating the spectrum that corresponds to two-time correlation data using fft

cgranade/qutip,anubhavvardhan/qutip,zasdfgbnm/qutip,cgranade/qutip,qutip/qutip,qutip/qutip,zasdfgbnm/qutip,anubhavvardhan/qutip