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

src/compute_improper_local.cpp

@@ -251,6 +251,6 @@ void ComputeImproperLocal::reallocate(int n)
 
 double ComputeImproperLocal::memory_usage()
 {
-  double bytes = nmax*nvalues * sizeof(double);
+  double bytes = (double)nmax*nvalues * sizeof(double);
   return bytes;
 }