Polish boost arith

This branch deal mainly with polishing and oxygenating  boost/simd/base/arithmetic
The main code novelties are the homogeneization of the code of "integer division"
(i)divxxx with xxx is floor, ceil, round and round2even
A fast_iround2even has also been added to use some sse2 intrinsics

All saturating functors as adds subs muls divs etc. has been given an alias of the form:
saturated_add, saturated_sub etc. 

also the new policy of not allowing integral parameters to fonctors naturally producing floating
has been enforced and remaining functions as rec for integral parameters acts as C integer division 1/x