[Cython] Another CEP: Parallel block

[Dag Sverre Seljebotn]

There's a (much shorter) proposal for a more explicit parallelism 
construct at

http://wiki.cython.org/enhancements/parallelblock

This is a little more verbose for the simplest case, but makes the 
medium-cases that needs work buffers much simpler, and is also more 
explicit and difficult to get wrong.

I am not sure myself which one I prefer of this and prange.

Justification for Cython-specific syntax: This is something that is 
really only useful if you can release the GIL *outside* of the loop. So 
I feel this is an area where a custom Cython solution is natural, sort 
of like "cdef extern", and the buffer access.

Since a similar pure-Python solution is rather useless, I also think 
there's less incentive for making something that works well in 
pure-Python mode.

Dag Sverre