! Invocation is:
!      dg2nc afile.dgXXXX [-im] [-o <output.nc>] [-v <variable>]
!    
! Flags:
!         -o <output.nc>   Send output to output.nc (If this option is 
!                          not used, the default output is to
!                          afile_XXXX.nc 
!         -v <variable>    Only get data for <variable>
!                          N.B. This can have multiple specifications
!         -i               Force writing over existing file
!         -m               Modify existing file rather than recreating
!
! Example usage:
!      dg2nc -v THBAR file1.dg0001 -o file2.nc
!  This invocation will output the variable VBAR from file1.dg0001 to
!  the output file file2.nc (Note all dimensional information is written
!  to file2.nc not just that required for THBAR.  file2.nc will be 
!  created if it doesn't already exist else the user will be prompted
!  whether to overwrite.  If the user wishes to modify file2.nc they
!  should use the -m flag (see next example).
!
!      dg2nc file1.dg0020 -i -m -v UBAR -v VBAR -o file2.nc
!  This invocation takes the variables UBAR and VBAR from the DaViE
!  file file1.dg0020 and modifies the existing netCDF file file2.nc 
!  N.B. The existing file2.nc must already contain the correct 
!  dimension information for the variables UBAR and VBAR
!