add unit/integrted tests for dpc

Can you document this a bit better.  I am having trouble match up what this says to what the code does.

I read this documentation as expecting a NxM array to be padded to (N + 2*pad)x(M + 2*pad) where image is in the center with a `pad` thick square of zeros around it.  The code seems to generate a (N*(2*pad + 1))X(M*(2*pad + 1)) array with the image in the middle with a (N*pad / M*pad) thick edge of zeros.

Sense you are shoving this image into an fft, you probably should also pad to final size to be a power of 2.