NEW YORK — Perhaps the Mets can rename their MRI scanner the “Jacob deGrom Tube.” The Mets ace on Monday had his third MRI in six weeks, with the latest image revealing “continued healing in the scapula,” according to a statement from the team. What does that mean for the two-time Cy Young award winner? DeGrom, who started throwing off flat ground about a week ago, has now received permission to take another step back and throw harder. For now, deGrom will build up his distance and velocity, with the next step being getting back on the mound. “Looked pretty clean,” GM Billy Eppler said of deGr…