Lazy data for GRIB.

Sorry. :unamused: I didn't like the "auto" prefix (it adds nothing!), but left it in the higher level API to keep it compatible - `auto_regularise` is part of the current public API of `iris.fileformats.grib.load_cubes()`. I guess I should either:
 - get rid of `auto_` from `messages_from_filename`, thus removing it completely from that module.
 - reinstate `auto_` everywhere in that module.

Thoughts?