Add plugin to get data of multiple peaks at event level

[HenningSE]

What does the code in this PR do / what does it improve?

This PR adds a plugin that propagates multiple peak properties to event level.

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

[coveralls]

coverage: 89.757% (-0.5%) from 90.215%
when pulling 7e49388 on multi_peak
into 1f80838 on master.

[HenningSE]

Need to wait for AxFoundation/strax#867 and #1406

[yuema137]

Hi, @HenningSE, thanks for this high-level PR. On top of the small comments that I made for the input parameters and docstring, I have a more general one - if we really need such a plugin here?

This plugin reads the relative time, position, and AFT of every peak within events. For a plugin named multiple_peak_data, other users may expect to get more information about those peaks but not just a few selected features. So I would suggest moving it to the analyses module if it's something needed for certain analyses. Basically, we just need to put a function to read from peaks and events data.