Move compute_center_times from straxen to strax

[dachengx]

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

Can you briefly describe how it works?

Besides moving, two small changes are made:

1. add dt / 2 to center_time
2. add time in the function (so center_time should be np.int64)

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: 89.181% (-0.09%) from 89.274%
when pulling 65e4bba on move_compute_center_times
into 956031e on master.