timegcm/trunk:
 - Add geometric height ZG as a mandatory field on secondary histories.
   ZG is added to 3d diagnostic fields (fields.F), and calculated by
   sub calczg in addiag.F. Note use of OX instead of O1 (as in tiegcm).
   Field ZG will be on secondary histories whether or not the user includes
   'ZG' in SECFLDS.
 - Removed array fzg, which was apparently allocated but not calculated or used.