added new for making Grib2Message with proper template classes.
added interp mthod to a g2io accessor on dataarrays
added Grib2GridDef as container for grid defs, for passing to interp
I couldn't have non-serializable objs in xarray_backend index, so added switch called data to grib2io.open to let me prevent data from being added to the Grib2Messages stored in _index.