Add Geopotential_isobaric alias for ECMWF grids

ECMWF grids only natively contain "geopotential", so to display "geopotential height" you must use a derived quantity.  However, currently "geopotential height" does not appear as a derived quantity for ECMWF data sources.  This simply adds a "Geopotential_isobaric" alias so the derived quantity will show up properly.

To test, load an ECMWF grib file as a data source.  If you see "3D grid" -> "Derived" -> "Height from Geopotential (Geopotential_isobaric)" as an option, the fix is working.  An example file for testing is here:

ftp://ftp.ssec.wisc.edu/pub/incoming/ecmwf.grb

This is a fix for McV inquiry 1129:
http://dcdbs.ssec.wisc.edu/inquiry-v/?inquiry=1129

Thanks!
Mike