gpdens:
   Routine extracts data from a gp-file for velocity 
   and computes various statistics for a specified depth
   depth level.

   The statistics are dumped to a netcdf file called 
   gpdens.nc. This file has vertical dimension as unlimited
   dimension, meaning that you can add more depth levels 
   to this file by running gpdens repeatedly. 

   Originally this routine was used for comparing model
   with observations, currently it only computes model
   Statistics. (there are pointers in the code on what
   needs to be changed). A lot of the statistics are useful
   on their own however. 

   ****************************************************
   Usage: gpdens rungen infile depthlevel
          rungen    : 3-letter ID of model run
          infile    : name of gp-file
          depthlevel: which depth to extract data from


   Output netcdf file contains statistics for observations (CM) and 
   model data (GP). In it current form these two are the same. For 
   variables dumped to the file, there should be a short comment 
   (ncdump -h gpdens.nc). 

   Based on code by Geir Evensen