[CF-metadata] grid_mapping

[Jonathan Gregory]

Dear All

Burkhardt and I have developed another suggestion for the names of the three
parameters specifying the rotated_latitude_longitude grid mapping:

  grid_north_pole_latitude
  grid_north_pole_longitude
  north_pole_grid_longitude

(Or it could be "rotated" in place of "grid" in each case, but maybe "grid"
is better because it's like the standard name.)

The first two are mandatory and specify the true latitude and longitude of the
north pole of the rotated grid. The third one is optional, default 0, and
specifies the longitude of the true north pole in the rotated grid.

Comments?

Jonathan