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

src/MISC/fix_orient_fcc.cpp

@@ -594,7 +594,7 @@ int FixOrientFCC::compare(const void *pi, const void *pj)
 
 double FixOrientFCC::memory_usage()
 {
-  double bytes = nmax * sizeof(Nbr);
+  double bytes = (double)nmax * sizeof(Nbr);
   bytes += (double)2*nmax * sizeof(double);
   return bytes;
 }