(Minneapolis, MN) —   Sun Country Airlines is adding 18 new nonstop routes for Minneapolis-St. Paul International Airport air travelers.   The Minneapolis air carrier also will begin seasonal nonstop flights to and from Duluth and Rochester international airports. Sun Country plans to add MSP flights to two more Florida destinations, St. Pete-Clearwater International Airport and Punta Gorda Airport, bring it to eight airports serving the Sunshine State.