Forgot to close the imw file while reading it.

       * src/ImwHelper.cc: fix a missing call to close().
Added a function to write imw files.

      * src/ImwHelper.cc: Here.
      * src/ImwHelper.hh: Here.
Added a shortcut function to WriteImage.

      * src/ImwHelper.cc: Here.
      * src/ImwHelper.hh: Here.