Read a variable in multiple NetCDF files, concatenate and write them to a single netcdf file.
% gpcat -v VAR [options] FILE1 FILE2...
-h, --help : print this message.
-v var, --variable var : variable name (required).
-s sfmt, --slice sfmt : slice,thinnng (optional).
-o file, --output file : output filename (optional).
: Default output filename is 'gphys.nc'.
2005/05/17 S Takehiro, created.