Allow start time to be set with EN_settimeparam

Add additional case for `EN_STARTTIME` to the `EN_settimeparam`
function.

src/epanet.c

@@ -1591,6 +1591,11 @@ int DLLEXPORT EN_settimeparam(EN_Project p, int param, long value)
         time->Qtime = value;
         break;
 
+    case EN_STARTTIME:
+        if (value < 0) return 213;
+	    time->Tstart = value;
+        break;
+
     default:
         return 251;
     }