more typecasts to double to avoid 32-bit int overflows when computing memory usage

src/compute_displace_atom.cpp

@@ -247,7 +247,7 @@ void ComputeDisplaceAtom::refresh()
 
 double ComputeDisplaceAtom::memory_usage()
 {
-  double bytes = nmax*4 * sizeof(double);
+  double bytes = (double)nmax*4 * sizeof(double);
   bytes += (double)nvmax * sizeof(double);
   return bytes;
 }