Pick out selection applying function

[dachengx]

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

Pick out the lines inside strax.utils.apply_selection where the selection is applied to be a stand-alone function parse_selection for other uses. Nothing changed functionally.

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: 91.507% (+0.005%) from 91.502%
when pulling 5418206 on apply_selection
into 67a7831 on master.