Conventions: CF-1.5 attribute loaded into cube as an attribute

[cpelley]

When a cube is written to netcdf, a global attribute it written to the netcdf file.

// global attributes:
        :Conventions = "CF-1.5" ;

I do not expect iris to load this global attribute in as an attribute on the cube sine this was not on the cube before writing it.

I propose we test for this conventions attribute and stop it from becoming an attribute on the cube.

I'm happy to do this myself.