8.18.15 mod_fft(z,optimize=0):

mod_fft $<$ Timing: $<$ Utilities.py

    NAM: mod_fft (function)
    DES: return squared module of the Fast Fourier Transform of an input sequence,
         limited to positive frequencies (assumes the input is real)
    INP: (flt array) z: 1D array on which to compute FFT
         (logical) optimize: if true, optimize the computation time by limiting the
                             input array to its first p elements, where p is the
                             largest power of 2 less than the number of elemenets
    OUT: (flt array) squared modules of FFT(z)