As European countries cope with a second wave of infections, data indicate that tourism has helped the coronavirus spread in popular destinations and travelers’ home countries. WSJ explains how summer travel has made it harder to control the pandemic. Photo: Damir Sencar/Agence France Press European countries are tightening cross-border travel restrictions as they try to rein in a surge in coronavirus infections and prepare for a possible further increase with the arrival of cold weather. Even as the thicket of travel restrictions across Europe grows, the European Union is seeking support for a coordinated regionwide approach that would make it easier for potential travelers to understand the increasingly complex rules. Brussels…