Add support for updating existing netcdf files

  Adds support for update mode.  This allows for one to open an existing netcdf file and add data to it.  I used the "a" flag for the mode to be compatible with the Scientific.IO.NetCDF library and the unidata C interface.

  This patch opens the file using the "r+" mode for new files and then calls _read() and _write if the mode is "a".  I did not do extensive testing.  But a quick check of opening an existing netcdf file and adding a new global attribute worked fine.