Value is overrated

Modify and fix NT2/Boost.SIMD so the value() fonction is not required anymore. It was 99% useless after the change makign sequence and fusion sequence automatically handled by tests but it was still lingering n various places. It is now removed and the protocol to handle primitive-like type is to go through the a more classical .value() extension point on said types.
This function was also supposed when T is a non-expression.
Maybe it would be useful to reintroduce a details::value for this, similar to details::raw?