Add WCS header

[albireox]

We need a system to add a WCS header group to the header model of an image. I think the best way is to add a new parameter to Exposure, Exposure.wcs. It would act similarly to Exposure.camera and be passed as part of the context.

The WCS can be defined manually by adding keywords whose values reference __wcs__, but we should also add a *magical group that will get expanded into a default astropy.wcs.WCS.to_header() group.