I/O Mixin

The I/O mixin, NDIOMixin, adds read and write methods that use the astropy I/O registry.

The mixin itself creates the read/write methods; it does not register any readers or writers with the I/O registry. Subclasses of NDDataBase or NDData need to include this mixin, implement a reader and writer, and register it with the I/O framework. See I/O Registry (astropy.io.registry) for details.