Update velocity example to have a run_all (or other) function.

[rosteen]

Running from slitlessutils.examples import velocity apparently runs some code, which results in 19 being printed out in the ipython session and the following plot being created/displayed:

Simply importing something in Python shouldn't run code like this, we should update this example.

EDIT: I took a look at the example code, didn't realize/forgot that this doesn't have a run_all method like starfield. Consider this issue to be about updating the velocity example code to have a callable function instead of a bare script.