add unit/integrted tests for dpc

It is just a warning.  I am pretty sure that this is the behavior
prescribed in the IEEE float spec.

On Tue Nov 25 2014 at 3:55:33 PM thegreatwall <notifications@github.com>
wrote:

> In skxray/dpc.py:
>
> > +    tx = np.fft.fftshift(np.fft.fft2(gx_padding))
> > +    ty = np.fft.fftshift(np.fft.fft2(gy_padding))
> > +
> > +    c = np.zeros((pad * rows, pad * cols), dtype=complex)
> > +
> > +    mid_col = pad * cols // 2.0 + 1
> > +    mid_row = pad * rows // 2.0 + 1
> > +
> > +    ax = 2 * np.pi * (np.arange(pad * cols) + 1 - mid_col) / (pad * cols * dx)
> > +    ay = 2 * np.pi * (np.arange(pad * rows) + 1 - mid_row) / (pad * rows * dy)
> > +
> > +    kappax, kappay = np.meshgrid(ax, ay)
> > +
> > +    c = -1j * (kappax * tx + w * kappay * ty)
> > +    div_v = kappax**2 + w * kappay**2
> > +    zero_arr = (div_v == 0)
>
> I would encounter a RuntimeWarning: divide by zero encountered in divide.
> If you think the warning is OK, I will bench mark these two.
>
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> <https://github.com/Nikea/scikit-xray/pull/177/files#r20894729>.
>