Removed undelete_below list (can be derived from undelete_index)
transformed undelete_backup and undelete_index into undelete_history list
Fixed merge_cell_below and merge_cell_above and an error in insert_element_at_index
wrote method merge_cells to be called by merge_cell_above and merge_cell_below
Added _fix_undelete_history method
Fixed a bug in merge_cells