Gotchas
-------

PyWavelets utilizes NumPy under the hood. That's why handling the data
containing None values can be surprising. Nones are converted to
'not a number' values:

    >>> import pywt
    >>> x = [None, None]
    >>> mode = 'sym'
    >>> wavelet = 'db1'
    >>> cA, cD = pywt.dwt(x, wavelet, mode)
    >>> print cA
    [           1.#QNAN]
    >>> print cD
    [           1.#QNAN]
    >>> print pywt.idwt(cA, cD, wavelet, mode)
    [           1.#QNAN            1.#QNAN]
