faster csv loading

ml_date.cpp

@@ -23,12 +23,16 @@ std::string date_to_string(const long datetime, const std::string format) {
 	return "invalid argument";
 }
 
+
+std::istringstream in_ss;
 long string_to_date(const std::string &datestr, const std::string &format) {
 	// format for example "%d.%m.%Y";
 
-	std::istringstream in{datestr.c_str()};
+	in_ss.clear();
+	in_ss.str(datestr);
+
 	date::sys_seconds tp;
-	in >> date::parse(format, tp);
+	date::from_stream(in_ss, format.c_str(), tp);
 	return tp.time_since_epoch().count();
 }