Per-object backend options

[tskisner]

Currently the selection of the backend format (e.g. HDF5) and all options to that backend (compression, chunksize, etc) are set at the volume level and applied to all groups within the volume. This is not ideal, since different kinds of data will require different options. Each object has all of the backend options stored in its location (backend_path object). We should devise a way to set these differently on a per-object basis. One possibility that would be easy to implement would be to use a selection string to specify which objects to set to a particular configuration.