Document the coarse-graining functionality #312
Description
Currently there is not really any documentation for how to use the coarse-graining functionality implemented in FV3GFS. This includes documentation from the perspective of a user (what is available, how to configure the coarse-graining resolution, how to configure the coarse-graining strategy, etc.) and the perspective of a developer (how to add new coarse-grained diagnostics). The coarse-graining code is reasonably mature at this point, so it might make sense to put some documentation somewhere.