Reading is too slow

[faroit]

currently stempeg loads stems by

• extracting stream-by-stream and convert to wav
  • load wav files and merge to tensor

For each stream extraction a os.call is used. For small files this could slows down stempeg loading significantly.

[faroit]

One attempt was made in #17, however it turns out to be even slower. I am open to better ideas to speed up the reading

[faroit]

using pipes from stdin improved reading speed significantly This is implemented in #28