Memory-mapped numpy arrays are not deleted if program quits suddenly

If the program quits suddenly (e.g. the Python interpreter crashes), the waveform numpy arrays stored on disk are not released. With large data sets, this causes disk space to run out quickly. To illustrate the scale of this problem, a 7.55 GB data set uses around 78 GB of disk space to store the waveform numpy arrays, none of which is released when the program exits ungracefully.