Use multiprocessing instead of fork.

[Joelgranados]

In timestream/manipulate/pipecomponets.py:(PlantExtractor) there was a comment that multiprocessing must be used instead of os.fork.

Putting it here so we don't loose track of the discussion. And I removed the comment from the code.

[Joelgranados]

Why should we prefer multiprocessing?

[Joelgranados]

I remember why I chose os.fork instead of multiprocessing:
We have to be hacky if we want to use multiprocessing with classes.
http://stackoverflow.com/questions/17172878/using-pythons-multiprocessing-process-class
http://www.rueckstiess.net/research/snippets/show/ca1d7d90

If you want to implement it in multiprocessing, that beast is all yours :)