scipy csr_matrix interop - Work in progress

[PhilipDeegan]

PR for #1665

I expect many changes, this is just to start with something, and see if tests pass.

[PhilipDeegan]

if I need to use some functions in addition to
https://github.com/pybind/pybind11/blob/master/include/pybind11/numpy.h#L203

Should I update this code, or add it to another version?
I'm still wrapping my head around this function pointer mapping business, any tips are appreciated.

[PhilipDeegan]

I assume the function codes are taken from here https://github.com/numpy/numpy/blob/master/numpy/core/code_generators/numpy_api.py

[PhilipDeegan]

I believe the csr_matrix is a part of "scipy" rather than "numpy".

Is it acceptable to add "scipy" to the test environments?