[SciPy-Dev] ENH: Extend peak finding capabilities in scipy.signal (#8264)

[Lars G.]

Dear SciPy devs,

I'd like to highlight my current pull request which extends SciPy's peak
finding capabilities in the signal module.
https://github.com/scipy/scipy/pull/8264
I've tried to address all feedback that was given (until now) in said
PR. However there are still some areas that would profit from a code
review and an additional pair of eyes.

Furthermore there are still some outstanding issues. It may be best to
discuss them here:

- There are still "Changes requested" which I have addressed or are
outdated. I don't know how to go about removing this flag.

- I think the Travis CI build fail is unrelated to my PR (something in
the stats module). Perhaps someone with more knowledge should confirm this.

- How to handle if invalid peaks are passed to the functions
peak_prominences and peak_widths? Is it okay to describe this as
undefined behavior in the documentation?

- argrelmax doesn't catch peaks that are wider than one sample. Decide
how to deal with this. I have implemented an alternative here which may
even be faster. But I plan to make a performance comparison to confirm
this. I have a feeling this should be addressed (if wanted) in a new
pull request.

- Should the new functionality be covered in the tutorial for
scipy.signal? I feel like that would be the best place to show more
complex examples which are out of place in the docstrings itself. If so,
I think a new PR would be the best place.

- The decision whether find_peaks should have a postfix like
"find_peaks_cwt" is still pending. If so which one?

Again, thank you all in advance for taking the time to review this PR
and any feedback given.

Best regards,
Lars