Time domain waveform to Fourier interface

[kazewong]

Currently ripple only supports Fourier domain waveform.

In preparing for the integration of time-domain waveforms such as the NRSurrogate waveforms, it would be nice to have an interface to convert time-domain waveforms into Fourier-domain waveforms.

The pseudo-code can look something like:

def waveform(f, params):
    t = f_to_t
    h_t = h(t, params)
    h_f = fft(h_t)
    return h_f