Store dtype in metadata of memmap

[cwmeijer]

This is about save_audio_features_to_memmap function in utils/preprocessing.py.

Would be way safer to store the dtype in the metadata of the memmap
so that we'd be sure to use the same dtype in dataset.py
If we don't use the same dtype in dataset.py, we'll end up with reading random
segments of the memory /!\ Not performed this way for LibriVox, so I did the same
for SpokenCOCO

@bhigy, I moved your code comment to this issue.