Migrate NTD to new behavior system

[svdavid]

A few things here:

• implement continuous backgrounds in the nAFC behavior paradigm. This may involve/require support for the new sound cards that allow asynchronous control of analog out channels.
• port the NTD code that runs on the go/no-go paradigm over to using nAFC.
  • increase the number of silent trials (ie, periods with no natural sound background)
• New foraging behavior as control for NTD behavior. Multiple ports active. One will randomly provide reward for a while until "depleted" and another port starts providing reward. Continuous natural backgrounds identical to NTD.