I would write `cosmic_ray` instead of `cosmicray` ... I don't know if that is an official PEP8 recommendation, but I find it more readable to separate words by underscores.
(Sorry if I'm commenting on things that don't actually change in this PR ... I've never looked at the `ccdproc` code and it's just more convenient for me to drop a comment here ... feel free to simply ignore comments you disagree with or don't want to address in this PR.)