GRIB2 grid definition template 5

This PR implements the "slim shady" grid definition template 5 for variable resolution rotated latitude/longitude.
Actually having discussed this further with @rhattersley we reckon that when note (3) uses the word _bounds_ it actually means something more like _the upper limit in octets for the list of longitudes and the list of latitudes_, not _where to go in the message to find lon and lat bounds values_.

As such it seems that *there are no bounds defined* for values in this template, so could you take back out the references to bounds from this function @bjlittle?

Would explain why there's no way to access them using gribapi though!