Revised grib load+save

Extra grib level-types translation, and grib <-> cf phenomenon translation.

These changes were originally driven by a requirement to form monthly averages from the ecmwf ERA climate series, input from Grib1 and output as Grib2.

Summary of changes:
*  Grib2<-->CF parameter translation now uses data from the "metadata translation project" 
  *  (file: fileformats/grib/grib_cf_map.py)
  * though the current version included has manual edits
*  Some Grib2 output now equates a "surface" type level with a "height=0.0" scalar coordinate (generated on input, .
*  Various new tests written and older ones enhanced
  
Note: also includes an updated  etc/cf-standard-name-table.xml