eriodic window, for use in spectral analysis.

Returns
-------
w : ndarray
    The window, with the maximum value normalized to 1 (though the value 1
    does not appear if `M` is even and `sym` is True).

Notes
-----
Flat top windows are used for taking accurate measurements of signal
amplitude in the frequency domain, with minimal scalloping error from the
center of a frequency bin to its edges, compared to others.  This is a
5th-order cosine window, with the 5 terms optimized to make the main lobe
maximally flat. [1]_

References
----------
.. [1] D'Antona, Gabriele, and A. Ferrero, "Digital Signal Processing for
       Measurement Systems", Springer Media, 2006, p. 70
       :doi:`10.1007/0-387-28666-7`.

Examples
--------
Plot the window and its frequency response:

>>> import numpy as np
>>> from scipy import signal
>>> from scipy.fft import fft, fftshift
>>> import matplotlib.pyplot as plt

>>> window = signal.windows.flattop(51)
>>> plt.plot(window)
>>> plt.title("Flat top window")
>>> plt.ylabel("Amplitude")
>>> plt.xlabel("Sample")

>>> plt.figure()
>>> A = fft(window, 2048) / (len(window)/2.0)
>>> freq = np.linspace(-0.5, 0.5, len(A))
>>> response = 20 * np.log10(np.abs(fftshift(A / abs(A).max())))
>>> plt.plot(freq, response)
>>> plt.axis([-0.5, 0.5, -120, 0])
>>> plt.title("Frequency response of the flat top window")
>>> plt.ylabel("Normalized magnitude [dB]")
>>> plt.xlabel("Normalized frequency [cycles per sample]")

)