GRIB2 grid definition template 5

This PR implements the "slim shady" grid definition template 5 for variable resolution rotated latitude/longitude.
That was pretty much the conclusion that I thought you had reached... 

I also can't see anything in the whatsnew for the most recent [updates to gribapi](https://software.ecmwf.int/wiki/display/GRIB/Latest+news) that say anything about accessing these bounds.