Delineate GRIB parameter table units

[lesserwhirls]

(for background, see #616)

The units, as found in GRIB parameter tables, do not adhere to any standard definition of units (for example, CF) and are essentially free text. As such, the units listed within a parameter table have widely varying degrees of usefulness. When translating GRIB messages into the CDM, the following steps should be taken to make the situation a little better:

1. Expose the unit, as described in a GRIB parameter table, using a new attribute, Grib1_Parameter_Units or Grib2_Parameter_Units (and soon, Grib3_Parameter_Units).
2. Using the units package, evaluate if the table unit is UDUNITS compatible.
   • If so, expose the table using using the units attribute.
   • If the table unit is not UDUNITS compatible, try to find an appropriate UDUNITS compatible substitute, and expose that using the units attribute.
     If a UDUNIT compliant version of the table unit cannot be found, do not assign a unit attribute.