Add re-entrancy checks to Nyquist

These changes below are paving the way for aud-do:

Nyquist Effect now detects self reentrancy and skips reentry if so.
Nyquist Effect now detects if another effect has run during effect processing, and does not send Nyquist
output if so.

Neither of these preparatory changes have an effect in absence of AUD-DO (or other function
that allows Nyquist effect to call out to Audacity functions).

src/commands/SelectCommand.cpp

@@ -207,6 +207,10 @@ void SelectTracksCommand::PopulateOrExchange(ShuttleGui & S)
 
 bool SelectTracksCommand::Apply(const CommandContext &context)
 {
+
+   // Count selection as a do-nothing effect.
+   // Used to invalidate cached selection and tracks.
+   Effect::IncEffectCounter();
    int index = 0;
    TrackList *tracks = context.GetProject()->GetTracks();