add a failure when discs are run with a solver other than forward euler

src/disc/evolve_disc.c

@@ -25,12 +25,18 @@ void evolve_disc(struct stardata_t * const stardata,
     Discdebug(1,"EV t=%g dt=%g\n",
               disc->lifetime/YEAR_LENGTH_IN_SECONDS,
               dtsecs/YEAR_LENGTH_IN_SECONDS);
+
+    if(stardata->preferences->solver != SOLVER_FORWARD_EULER)
+    {
+        Exit_binary_c(BINARY_C_ALGORITHM_OUT_OF_RANGE,
+                      "The discs algorithm has not been tested with solvers other than Forward-Euler. Please only use this solver.");
+    }
     
     disc->first = disc->first==TRUE ? TRUE : Boolean_(Is_zero(disc->lifetime));
 
     disc->dT = 0.0;
     Clear_disc_feedback(disc);
-    
+
     if(Disc_is_disc(disc)) 
     {
         /*