(rasqal_new_query_results_from_string): Added
(rasqal_new_query_results_from_string): Guess formatter from data

If @formatter is NULL guess it using input data via
rasqal_world_guess_query_results_format_name()
(rasqal_new_query_results_from_string): Always guess formatter from data

Remove formatter arg.
(rasqal_new_query_results_from_string): Make string_len optional.