back to SSE

Given the current level of optimization and that AVX will be not used in production in 2015 we revert to the SSE version for all vectors.
(also removed an obsolete "pretended to be" SSE-specific code)

Purely technical, no regression expected.

Speed up expected and observed in few routines using double precision four vectors.
for future reference (we may need them for arm for instance)
AND we will have to change the whole logic as soon as we will need to support GCC 5.0