Fix removed use of LENGTH on S4SXP

* src/rmatio.c (map_vec_len): Removed usage of LENGTH on S4SXP. Added
  check of S4 class. Updated doxygen documentation.
Fix removed use of LENGTH on S4SXP

* src/rmatio.c (check_ragged): Removed usage of LENGTH on S4SXP. Added
  check of S4 class. Updated doxygen documentation.
Cell test case 16 on fewer lines to show all info