fix flat rate particle mortality

modules/slimParticles/dgParticleTracker2D.cpp

@@ -843,7 +843,7 @@ int dgParticleTracker2D::isDead(double t)
     case 0:
       return 0;
     case 1:     /* Const. mortality rate */
-      mortRatePerDt = _dt * _mortRate_max/(3600.0*24.0);
+      mortRatePerDt = 1-pow(1-_mortRate_max, _dt/(3600*24));
       if ( ((randoraw()/2)+0.5) < mortRatePerDt ) return 1;
       else return 0;
     case 2:     /* Weibull mortality rate */

modules/slimParticles/dgParticleTracker2D.h

@@ -370,6 +370,9 @@ public:
   void addMortality(int type, double rateMax, double weibullLambda = 0, double weibullNu = 0) {
     _mortFlag = true;
     _mortType = type;
+    if (type != 1) {
+      Msg::Warning("Weibull mortality implementation may be wrong\n");
+    }
     _mortRate_max = rateMax;
     _mortLambda = weibullLambda;   //(Connolly & Baird 2010)lambda = 1/b(Pinder et al. 1978)
     _mortNu = weibullNu;       //(Connolly & Baird 2010)nu = c(Pinder et al. 1978)