GRIB2 grid definition template 5

This PR implements the "slim shady" grid definition template 5 for variable resolution rotated latitude/longitude.
However note (3) for GDT 3.5 says
> For the list of Ni longitude bounds and Nj latitude bounds at the end of the section: ii = 60 + 4Ni and jj = 60 + 4Ni +4Nj
Doesn't deal with the problem of accessing these via coded keys though!