add unit/integrted tests for dpc

I did change the default values for the kwargs. And interestingly, I benchmark test_dpc_end_to_end with dpc_fit() and with minimize() directly:
image size, dpc_fit, minimize
2-by-2: 21.8ms, 22ms
20-by-20: 2.15s, 2.24s
60-by-60: 19.4s, 20.1s
dpc_fit() doesn't seem to slow it down. So I still want to reserve dpc_fit().