Calculate area_fraction_top, center_time and median_time at peaklet level

[dachengx]

What is the problem / what does the code in this PR do

Add area_fraction_top, center_time, and median_time to peak_dtype. They will be calculated by compute_properties.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

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?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 89.201% (+0.02%) from 89.181%
when pulling db8356c on add_center_time_median_peaks
into bfb947d on master.