DOC: Improve examples in signal.find_peaks with ECG signal

[lagru]

This PR should improve the example section of scipy.signal.find_peaks documentation. If you feel that something could be explained better or is confusing please let me know.

My thoughts behind this are:

• Use scipy.misc.electrocardiogram as the demo signal to move the examples a little closer to real world use cases.
• Demonstrate different use cases: half open and dynamic intervals, combining conditions.
• Nearly all conditions (expect threshold) are covered with at least one example and one plot. I didn't find a meaningful use for threshold that has an explanatory value.

[lagru]

[Rendered docstring]

By the way, is there an easy method to access the documentation created by CircleCI? The only way I know is to search for the correct link in the returned JSON here:
https://circleci.com/api/v1.1/project/github/scipy/scipy/BUILDNUMBER/artifacts

[larsoner]

Follow the CircleCI build link from the GitHub status at the bottom of the PR, and make sure you're signed in (via GitHub auth) to CircleCI. An "Artifacts" tab should show up.

[lagru]

Ah, thank you! I wasn't sure if I was missing the rights to access the artifacts.