'**SUMMARY**
Get the numpy array of the filter
**RETURNS**
numpy array of the filter.
**EXAMPLE**
>>> notch = DFT.createNotchFilter(dia1=200, cen=(200, 200),
size=(512, 512), type="highpass")
>>> notch.getNumpy()'

def getNumpy(self):

    if isinstance(self._numpy, type(None)):
        if isinstance(self._image, type(None)):
            warnings.warn("Filter    doesn't    contain    any    image")
        self._numpy = self._image.getNumpy()
    return self._numpy
