retrieves the callback routines set for working with file images opened with the file access property list
The callbacks must have been previously set with
in the file access property list.
Upon the successful return of
H5P_SET_FILE_IMAGE_CALLBACKS, the fields in the instance of the
H5_file_image_callbacks_t struct pointed to by
callbacks_ptr will contain the same values as were passed in the most recent
call for the file access property list