Lookup for dynamic sky fiber placement

[dstndstn]

This is the code we're using in fiberassign to do the lookup for good sky locations in the "skybricks" data product. Stephen requested it go in desitarget so he can use it in the spectro pipeline.

[coveralls]

Coverage decreased (-0.3%) to 59.213% when pulling 258973e on dynamic-sky into 073e430 on master.

[geordie666]

@dstndstn: This PR looks fine, and it's OK to merge it. Before you do, though, make sure to update the changes documentation and the API for read-the-docs.

[sbailey]

Thanks; I also tested that this works as expected, and updated the changes.rst and api.rst so that @dstndstn can focus on other topics. I'll merge after tests re-pass.

[dstndstn]

Wow, thanks @sbailey!