fix(elecraft): the ATU is switch 19, not 20

SWT20 was written from memory of the reference rather than from Table 7,
and 20 is not an ATU switch at all — it would have pressed something else
on a real K3. An operator read the table out for us: switch 19 is the ATU
row, TAP for a tuning cycle and HOLD for tuner in line or bypassed.

Adds the HOLD as its own ATU button, because on the radio they are two
different things: tuning is a cycle you start, bypassing is a state you
leave it in.

Power takes 0-110 W on an Elecraft, per the reference — a K3 makes a
little over its rated output — while a Kenwood keeps 200.

app_k3.go

@@ -91,6 +91,12 @@ func (a *App) SetKenwoodTX(on bool) error {
 	return a.kenwoodPanelDo(func(k cat.KenwoodPanelController) error { return k.SetKenwoodTX(on) })
 }
 
+// ToggleKenwoodATU puts the tuner in line or bypasses it (the HOLD of the same
+// front-panel switch the tune uses).
+func (a *App) ToggleKenwoodATU() error {
+	return a.kenwoodPanelDo(func(k cat.KenwoodPanelController) error { return k.ToggleKenwoodATU() })
+}
+
 func (a *App) TuneKenwoodATU() error {
 	return a.kenwoodPanelDo(func(k cat.KenwoodPanelController) error { return k.TuneKenwoodATU() })
 }