norm_inf to answer issue #758

just norm_inf for now

 that is :
   max(abs(a)) for a vector
   max(sum(abs(a), 2)) for a matrix
 
 norm_inf can be used as
 norm_inf(a)
 norm_inf(a, nt2::vector_)
 norm_inf(a, nt2::matrix_)
 
 the second parameter can be used to suppose (with vector_) a is always a big column vector
 or (with matrix_ )  a  is always  matrix.

Joel does that fit ?
Don't think it's needed here