Prevent negative values in reactor state

dependencies/reactor_class.js

@@ -65,7 +65,7 @@ class Reactor_CSTR {
 
         const dC_total = math.multiply(math.add(dC_in, dC_out, r, t_O), time_step);
 
-        this.state = math.add(this.state, dC_total);
+        this.state = math.abs(math.add(this.state, dC_total)); // make sure that concentrations do not go negative
         return this.state;
     }
 }