Provide an interface for scipy.fft

[peterbell10]

The upcomping SciPy 1.4.0 release will include a new subpackage scipy.fft that supercedes scipy.fftpack. This new interface matches the behaviour of numpy.fft closely including real to complex rffts but is different in a few ways (e.g. dtype casting).

This interface also includes a new backend mechanism (scipy/scipy#10383) that would allow mkl_fft to implement the scipy.fft interface without any monkey patching. e.g. from pyFFTW/pyFFTW#269:

import scipy.fft
scipy.fft.fft([1])  # Calls scipy's own implementation

from pyfftw.interfaces import scipy_fft
scipy.fft.set_global_backend(scipy_fft)
scipy.fft.fft([1])  # Calls into pyfftw

See scipy_fft.py for what needs to be done on the mkl_fft side to support this.

It's also unfortunate that the name mkl_fft._scipy_fft is already taken. Any ideas on how to move forward with this?