Allow scipy_fft module to be used as a scipy fft backend

[bombaci-vsc]

This is an approach for addressing #170. It makes mkl_fft.interfaces.scipy_fft also work as a SciPy FFT backend.

The _scipy_fft module can already be used as a backend, but it is only accessible via an underscored module.
mkl_fft.interfaces.scipy_fft is the public interface, but it isn't importing the dunder properties needed by scipy backends.

I've also updated the README to include an updated version of the example found in the v1.1.0 changelog.

[vtavana]

Thank you, @bombaci-vsc for creating this PR. You are right about the issue you mentioned.

However, we are in the process of fixing this issue. We have this open PR which brings mor changes to the structure of the package. And on top of it, we have a branch (that we will merge later) which addresses the issue you mentioned. In this branch, we have also included a more comprehensive explanation about using mkl_fft as a backend.

[bombaci-vsc]

Good to hear. I did see the open PR and noticed that _scipy_fft.py was going to move. That's what motiviated me to open this PR to encourage moving the backend interface into interfaces.scipy_fft. I'm glad to hear that's the path you're already on.

Do you anticipate merging the add-readme-interfaces branch before making a relase with #168? Or will the backend module move once more before ending up in interfaces.scipy_fft?