fix GeoData export

[aelligp]

This PR fixes cross_section_surface to export the correct AbstractGeneralGrid type rather than only GeoData:

    if isa(S,GeoData)
        Data_profile = GeoData(Lon, Lat, depth_intp, fields_new);
    elseif isa(S,CartData)
        Data_profile = CartData(Lon, Lat, depth_intp, fields_new);
    else
        error("still to be implemented")
    end

[boriskaus]

thanks for the fix!

If you add a test for this, CodeCov will be happy