add unit/integrted tests for dpc

To invert the sign of gx before phase reconstruction. Originally the code looks like: 
gx *= -len(ref_fx) * pixel_size[0] / (lambda_ * focus_to_det)
Now it looks like:
gx *= len(ref_fx) * pixel_size[0] / (lambda_ * focus_to_det)
if invert:
    gx = -gx
I talked with Hanfei and he implied that for some datasets, there might be no need for the negation correction.