MSVC Compilation errors.

From a user:

I work for DxO (still), you should have seen my commits. yesterday I upgraded to Visual Studio 15.8.0 and found Pylene failed to compile.

Though the fix was easy (I had to change count.hpp, infsup.hpp, max.hpp, mean.hpp and min.hpp), Nicolas would prefer to get the changes from you.

To fix, I commented out the following lines:

namespace extractor {

  template <typename A>
  inline
  auto
  count(const Accumulator<A>& acc)
    -> decltype(extract(exact(acc), std::declval< features::count<> > ()));

}

namespace extractor {

  template <typename A>
  auto
  inf(const Accumulator<A>& acc)
    -> decltype( extract(exact(acc), std::declval< features::inf<> > ()));

  template <typename A>
  auto
  sup(const Accumulator<A>& acc)
    -> decltype( extract(exact(acc), std::declval< features::sup<> > ()));

}

namespace extractor {

  template <typename A>
  auto
  max(const Accumulator<A>& acc)
    -> decltype( extract(exact(acc), std::declval< features::max<> > ()) );
}

namespace extractor {

  template <typename A>
  inline
  auto
  mean(const Accumulator<A>& acc)
    -> decltype(extract(exact(acc), std::declval< features::mean<> > ()));

}

namespace extractor {

  template <typename A>
  auto
  min(const Accumulator<A>& acc)
    -> decltype( extract(exact(acc), std::declval< features::min<> > ()) );

}

Could you please fix the library? Thanks in advance.