added kAudioFileFlags_EraseFile in AudioFileWriter

[ghost]

the example code for writing to an audio file worked on the first attempt but on subsequent calls, ExtAudioFileCreateWithURL exited with the error -48 duplicate filename. Using kAudioFileFlags_EraseFile fixes this.

Edit / additional info for second push: fixed another small bug in the AudioFileWriter init method. When passing numChannels:1, it still produces a stereo file, this is due to the line

AudioStreamBasicDescription outputFileDesc = {44100.0, kAudioFormatMPEG4AAC, 0, 0, 1024, 0, 2, 0, 0};

where currently the 7th parameter (for num channels) is hardcoded to two. Changed this to

AudioStreamBasicDescription outputFileDesc = {44100.0, kAudioFormatMPEG4AAC, 0, 0, 1024, 0, thisNumChannels, 0, 0};