added access to sb by their names
added add_mapping method
added retrieve_sb_by_name method, which is used to map sb with their names, as some methods accept only sb, while others can work with both just string names or full synteny blocks object with respective names
added fix for get_mapping_and_offset_for_sb method, as it was passing string value, while get_two_closest_mappings_and_relative_position was expecting SyntenyBlock object
added add_mapping method, which accepts string values
added check/fix for the get_mapping_set_for_sb, as it shall pass nonzero argument as r_offset to get_mapped_set for the purpose of slicing in get_mapped_set method
added check for the cases, when None value is passed