Changes owing to gribapi version upgrade
Review actions: gribapi version now more strictly determined
made grib_get kwarg "type" an arg for v1.12.1 compatibility
Pinned ecmwf_grib to v1.12.1
Merge pull request #1250 from dkillick/update_gribapi_v1-12-1

Update gribapi dependency to v1.12.1
Move _messages_from_filename to _GribMessage.
On-demand value lookup.