ToPrefix generates non-SMTLIB-compliant strings

[soonhokong]

• A negative constant -c should be formatted as (- c).

  dreal4/dreal/symbolic/prefix_printer.cc

  Lines 50 to 58 in db12dd0

  	ostream& PrefixPrinter::VisitConstant(const Expression& e) {
  	return os_ << get_constant_value(e);
  	}
  	ostream& PrefixPrinter::VisitRealConstant(const Expression& e) {
  	const double mid{get_lb_of_real_constant(e) / 2.0 +
  	get_ub_of_real_constant(e) / 2.0};
  	return os_ << mid;
  	}