[SciPy-Dev] previous/next for interp1d

[Dieter Werthmüller]

Dear Devs,

I wanted to put your attention to 
https://github.com/scipy/scipy/pull/8572, which includes a 
`previous`/`next` feature to `interp1d`.

Only by making the pull request I realized that the same feature got 
already a pull request some 1.5 years ago, 
https://github.com/scipy/scipy/pull/6718. However, it did not proceed 
any further.

The two pull requests are slightly different, but it is beyond me to say 
which one is better or more adequate. However, I hope that you will 
include one or the other, as there are now already two pull requests for 
the same feature.

I included a few basic tests, copied and adjusted from the `nearest` 
interpolation. Let me know if more are required.

Regards,
Dieter