BF: fix for soundfile float division bug in duration

As it turns out it was inefficient to create sf.info(filename) for
this anyway. We now do tha calculation ourselves which means only
opening the file once

psychopy/sound/backend_sounddevice.py

@@ -360,7 +360,7 @@ def _setSndFromFile(self, filename):
        self.sampleRate = f.samplerate
        if self.channels == -1:  # if channels was auto then set to file val
            self.channels = f.channels
        info = sf.info(filename)  # needed for duration?
        fileDuration = float(len(f))/f.samplerate  # needed for duration?
        # process start time
        if self.startTime and self.startTime > 0:
            startFrame = self.startTime*self.sampleRate
@@ -371,19 +371,18 @@ def _setSndFromFile(self, filename):
        # process stop time
        if self.stopTime and self.stopTime > 0:
            requestedDur = self.stopTime - self.t
            maxDur = info.duration
            self.duration = min(requestedDur, maxDur)
            self.duration = min(requestedDur, fileDuration)
        else:
            self.duration = info.duration - self.t
            self.duration = fileDuration - self.t
        # can now calculate duration in frames
        self.durationFrames = int(round(self.duration*self.sampleRate))
        # are we preloading or streaming?
        if self.preBuffer == 0:
            # no buffer - stream from disk on each call to nextBlock
            pass
        elif self.preBuffer == -1:
            # no buffer - stream from disk on each call to nextBlock
            sndArr = self.sndFile.read(frames=len(self.sndFile))
            # full pre-buffer. Load requested duration to memory
            sndArr = self.sndFile.read(frames=int(self.sampleRate*self.duration))
            self.sndFile.close()
            self._setSndFromArray(sndArr)