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

src/improper.cpp

@@ -388,7 +388,7 @@ void Improper::ev_tally(int i1, int i2, int i3, int i4,
 
 double Improper::memory_usage()
 {
-  double bytes = comm->nthreads*maxeatom * sizeof(double);
+  double bytes = (double)comm->nthreads*maxeatom * sizeof(double);
   bytes += (double)comm->nthreads*maxvatom*6 * sizeof(double);
   return bytes;
 }