pace-util: quantity attrs inaccessible

[brianhenn]

pace.util.Quantity has an attribute _attrs that isn't accessible via instantiation or public setter (though it has a getter). Because Quantity is used in the fv3net integration for writing zarrs, this means that fv3net is currently using private API to write a zarr with array attributes. In the spirit of making the fv3net -pace.util integration more robust, seems like we should add a setter for Quantity._attrs, or go the other way and get rid of Quantity._attrs entirely.

[mcgibbon]

I think we originally had an attrs argument to the constructor for Quantity, but removed it for reasons I don't remember. We could add it back if we want to use it for writing to zarr.