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

src/KSPACE/pair_tip4p_long.cpp

@@ -523,7 +523,7 @@ void *PairTIP4PLong::extract(const char *str, int &dim)
 
 double PairTIP4PLong::memory_usage()
 {
-  double bytes = maxeatom * sizeof(double);
+  double bytes = (double)maxeatom * sizeof(double);
   bytes += (double)maxvatom*6 * sizeof(double);
   bytes += (double)2 * nmax * sizeof(double);
   return bytes;