Limit atom energy change in fix dt/reset

Allow limiting of the maximum energy change of an atom in
fix dt/reset in addition to the existing distance limit.
Useful especially for high-energy irradiation.

src/fix_dt_reset.h

@@ -37,8 +37,8 @@ class FixDtReset : public Fix {
  private:
   bigint laststep;
   int minbound,maxbound;
-  double tmin,tmax,xmax;
-  double ftm2v;
+  double tmin,tmax,xmax,emax;
+  double ftm2v,mvv2e;
   double dt,t_laststep;
   int respaflag;
 };