Change units of output-nc to always be the recommended values

[aleksandervines]

CF recommends that the units of latitude and longitude are set to degrees_north or degrees_east. One reasoning they give is that "Udunits package does not recognize the directionality implied by the "north" part of the unit specification".

http://cfconventions.org/cf-conventions/v1.6.0/cf-conventions.html#latitude-coordinate

I therefore suggest that, if the user selects south or west, rosetta switches the sign of the data and produce output netcdf file with north/east values, as recommended.

[aleksandervines]

Also just noticed that in the variable specifications, degree_north is used and in platform specification, degrees_north. Both are recognized and accepted by udunits, but wouldn't it be an idea to stay consistent? :)