add typecast to avoid (unlikely but possible) integer overflows.

this will silence a lot of static code analysis warnings

src/compute_centro_atom.cpp

@@ -437,6 +437,6 @@ void ComputeCentroAtom::select2(int k, int n, double *arr, int *iarr)
 double ComputeCentroAtom::memory_usage()
 {
   double bytes = nmax * sizeof(double);
-  if (axes_flag) bytes += size_peratom_cols*nmax * sizeof(double);
+  if (axes_flag) bytes += (double)size_peratom_cols*nmax * sizeof(double);
   return bytes;
 }