move precomputation for factor_sqrt to individual Pair::compute() functions

the special_lj values may be changed for individual hybrid sub-styles
with pair_modify pair special. thus the factor_sqrt[] array may have
incorrect values when computed during Pair::init_style().

src/DPD-BASIC/pair_dpd_tstat.cpp

@@ -48,6 +48,10 @@ void PairDPDTstat::compute(int eflag, int vflag)
 
   ev_init(eflag,vflag);
 
+  // precompute random force scaling factors
+
+  for (int i = 0; i < 4; ++i) special_sqrt[i] = sqrt(force->special_lj[i]);
+
   // adjust sigma if target T is changing
 
   if (t_start != t_stop) {