Factor exception handling code
Start adding initialisation and finalisation functions to each parallel combinator.
The initialisation function will receive the number of the core it is running on.
added init to mapper
Use redirect as default initialisation function
Adding init and finalize parameters to the main functions.
Added init and finalize parameters to the internal functions
Call init and finalize in the workers and in the finish function of setup_chan
Following advice from @UnixJunkie, initialisation function
defaults to doing nothing.

We need to expose a redirect function instead, helping
who may want to use it without rewriting it from scratch.
Remove redirection flag, and introduce a redirect helper function
Updated utils.ml in tests to allow passing init functions