Palestinian medics said at least 50 Palestinians were killed when Israeli air strikes hit a densely populated refugee camp in north Gaza on Tuesday, as Israeli ground forces battled Hamas gunmen based in a sprawling tunnel network.   Israeli military spokesperson Lieutenant Colonel Richard Hecht, speaking to CNN, confirmed the Israeli strike on Jabalia refugee camp and said it targeted “a very senior Hamas commander in that area.” He added: “We’re looking into it and we’ll be coming out with more data as we learn what happened there.”   U.N. and other aid officials said civilians in the besieged Palestinian enclave were engulfed by a public health catastrophe, with hospitals struggling to treat snowballing casualties as electricity supplies peter out.   Israeli Prime Minister Benjamin Netanyahu dismissed international calls for a “humanitarian pause” in fighting to enable emergency aid deliveries to civilians suffering from critical shortages of food, medicine, drinking water and fuel....