Filter: protect against extremely low notch filter frequencies

an incorrectly configured ESC telemetry source can lead to a notch
running at very low frequencies. A simple example is a lua script like
this:

function update()
   esc_telem:update_rpm(12, 0, 0)
   return update, 10
end
return update()

where motor 12 is unused.

with that script in place we get a 1.0078 Hz filter which leads to
massive phase lag and a crashed aircraft

this is a safety protection. We should also try to find out why the
INS_HNTCH_FREQ lower limit is not working

libraries/Filter/NotchFilter.cpp

@@ -104,7 +104,7 @@ void NotchFilter<T>::init_with_A_and_Q(float sample_freq_hz, float center_freq_h
 template <class T>
 T NotchFilter<T>::apply(const T &sample)
 {
-    if (!initialised || need_reset) {
+    if (!initialised || need_reset || _center_freq_hz <= 2.0) {
         // if we have not been initialised when return the input
         // sample as output and update delayed samples
         signal1 = sample;