avoid copying data to Python but using numpy::frombuffer

[rijobro]

Just an idea.

We can use numpy.frombuffer() to share data between C++ and python (e.g., for ProjDataInMemory). I think all that needs to be done is pass the address of begin to the python level. No idea how to do that with your wrapper, though, @evgueni-ovtchinnikov. Any ideas?