[eigen] vectorization of sum

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi,

I looked at general vectorization of redux but this is not very convenient...
- in the general case it would be better done as a separate method packetRedux
- this would require the size to be a multiple of packetsize even in dynamic 
case
- this is not the same as vectorizing redux when possible (return type is a 
packet instead of a scalar)
- the final reduction step producing a scalar is not available in general, 
only in the special case of the sum is it available as ei_predux...

So instead, I'm now looking at splitting sum away from Redux, and vectorizing 
it.

Cheers,
Benoit

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