Proposed fix for #1167.

Fixes saving cubes with reserved attribute names to netCDF.
Added test for saving cube to netcdf with clashing reserved attribute.
Use getncattr when loading global attributes.

Avoid a situation where an attribute whose name clashes with a
netCDF4.Dataset attribute (e.g., dimensions) will end up with the
value of the netCDF4.Dataset attribute instead of the actual
attribute from the netCDF file.
Add a package init file for the cf tests.

Otherwise the tests aren't run!
Merge pull request #1326 from ajdawson/setncattr

Fix for netCDF4 reserved attribute names.