accept data files without zlo/zhi. use -0.5/0.5 as implied in the docs

src/read_data.cpp

@@ -349,6 +349,10 @@ void ReadData::command(int narg, char **arg)
   triclinic = 0;
   boxlo[0] = boxlo[1] = boxlo[2] = 0.0;
   boxhi[0] = boxhi[1] = boxhi[2] = 0.0;
+  if (domain->dimension == 2) {
+    boxlo[2] = -0.5;
+    boxhi[2] =  0.5;
+  }
   keyword[0] = '\0';
 
   nlocal_previous = atom->nlocal;