I'm loving the context management protocol usage within the new ```SaveManager``` class. Very nice!

However, the saver feels disjointed some what from the rest of the file now. Would you consider promoting the associated module level saver functions to be methods of this class now? To me the two are highly coupled. The ```SaveManager``` class can't operate without, for example, the module level ```_create_cf_variable``` function. Why not fold all these functions into the class to remove this dependency. These module level functions are private to the module, so why now are they living at the module level?

See
 * ```_cf_coord_identity()```
 * ```_create_bounds()```
 * ```_cube_netcdf_variable_name()```
 * ```_coord_netcdf_variable_name()```
 * ```_create_cf_variable()```
 * ```_create_cf_cell_methods()```
 * ```_create_cf_grid_mapping()```
 * ```_create_cf_data_variable()```
 * ```_increment_name()```

All of which could be folded into the class and re-factored to share any shared data.