findlayer:
   Produces data for layers above and below the variable tresholds. 
   Top interface of this layer and thickness of layer is dumped in 
   a netcdf file named LAYERTHK.nc.      
   Note that only the first layer from the surface which matches the
   criterion is dumped in the netcdf file.

   Usage: findlayer VARIABLE minimumvalue maximumvalue file 
     where "VARIABLE" is 3D var in  file (eg saln)
     where minimumval is minimum of variable range
     where maximumval is maximum of variable range
     where VALUE is treshold value            
   NB: specifying th3d as variable will use density as variable

   Example: 
      findlayer saln 35 35.2 FORDAILY_2008_302_2008_318.a

   Outputs netcdf file with layer thickness and top surface of layer 
   with salinity > 35  and salinity < 35.2

Knut Liseter, 23. November 2008