Changed how the t_start value is ensured to be equal to the last end

point (if given) using a tolerance of 1e-15. This fixes an
issue where the t_start value and the end point are the same
but "==" operator returns false due to machine precision error and the
simulation fails to run.

bng2/Perl2/BNGAction.pm

@@ -421,13 +421,14 @@ sub simulate
 
     # Set start time for trajectory
     my $t_start;
+    my $tol=1e-15;
     if ( defined $params->{t_start} )
     {
         $t_start = $params->{t_start};        
         # if this is a continuation, check that model time equals t_start
         if ($continue)
         {
-            unless ( defined($model->Time)  and  ($model->Time == $t_start) )
+            unless ( defined($model->Time)  and  (abs($model->Time - $t_start) < $tol) )
             {  return "t_start must equal current model time for continuation.";  }
         }
     }