Fix around implementation

It is only possible to have a numexpr for floating point expressions.
On Sun, Oct 05, 2014 at 05:02:25AM -0700, pbrunet wrote:
> In pythran/pythonic/numpy/around.hpp:
> >                  return proxy::rint()(a * nt2::pow(typename types::
> numpy_expr_to_ndarray<E>::type::dtype(10),decimals)) / nt2::pow(typename
> types::numpy_expr_to_ndarray<E>::type::dtype(10), decimals);
> > +              else
> > +                return proxy::rint()(a / nt2::pow(typename types::
> numpy_expr_to_ndarray<E>::type::dtype(10),-decimals)) * nt2::pow(typename
> types::numpy_expr_to_ndarray<E>::type::dtype(10), -decimals);
> I don't get it. If decimal is positive, nothing have to be done for integer.
> Also, why the formula should change? It compute exactly the same thing no? I
> mean, dividing by a negative exponant and multiply by its positive value is the
> same.
We are only handling integers, and nt2 turns negative power to 0, thus
this behavior. however, I'll update the PR, I just note that around(int,
postitive) is always the identity!

> Also, I know it is only an optimisation but look at : pbrunet@20be15d
> I think you can do the same for this case.
Smarty! I'll leave it for another one though.