Add gbox option to cosmicray_lacosmic

I started playing around with this function yesterday and ran into at least one case where the ability to grow the cosmic ray mask a bit is necessary to fully eliminate CRs without also catching noise. Will post a link to a notebook demonstrating what I was hitting later today.

Start by locating the cosmicray_lacosmic function and reading how its options control cosmic-ray mask generation. Use the issue's promised notebook or a comparable case to understand the need for gbox. Done means the function accepts this option and can grow the mask enough to remove cosmic rays without also catching noise.

Written by the indexing model from the issue text.