Tiff: added octave_tiff_handle class to wrap the Tiff file pointer * __tiff__.cc (octave_tiff_handle): implemented a new class octave_tiff_handle as a child of octave_base_value to be a wrapper for the Tiff file handle to be passed to Octave instead of passing a pointer around. * __tiff__.cc (F__open_tiff__, F__tiff_close__): modified handling TIFF file pointer to use the new class (Other internal functions were modified as well). * Tiff.m: removed checking for closed file, it is now handled internally.