add unit/integrted tests for dpc

@thegreatwall , I think this might be a little contradict to what we discussed before. But still please add a function called dpc_runner in dpc.py with format below. 
```python
def dpc_runner(start_point, pixel_size, focus_to_det, rows, cols, energy, roi, pad, w, pad_pixels, solver, image_size, ref_image, image_sequence):
```

This function performs as a main controller to run the whole dpc. Then write a end_to_end test for this main controller.