GRIB2 grid definition template 5

This PR implements the "slim shady" grid definition template 5 for variable resolution rotated latitude/longitude.
I think that his template is so shiny new that the coded keys just don't exist in the `gribapi`. There's certainly no data out there for this template that I know of!

The spec tells us where the bounds data lives at the octet level within the section, but I can't find **how** to get to it through the `gribapi`. We'll just have to live with this uncertainty at the moment until the `gribapi` and the community catches up with the spec.