Integrate Cupy_Custer in the cuda_phase-retrieval example

[Simeon Ehrig]

Integrate the cupy_ref_custom caster in the cuda_phase-retrieval code. This task should test the cupy_ref_custom caster in a real world code and show, what is missing.

Following Python code should be possible:

# ...
ph = cuda_phase-retrieval(image, masks, mode, beta, randoms)
ph.iterate(10)
# should return a cupy_ref
result = ph.get_result()

# create image from result and display it

More Python-Bindings can be added.

[Simeon Ehrig]

Done in #22