[bugfix] Don't implent operator<< templated wrt the stream

When implementing operator<<(S& , const T&) to provide character stream support for a custom type T, the overload should be specific to S=std::ostream and not be templated wrt S. Otherwise this may lead to ambiguous overloads downstream. To give two examples for problematic cases:

Here we defined an overload

template<class Stream, class T, std::size_t N>
inline Stream& operator<<(Stream&, const std::array<T,N>&)

If a downstream module wants to defines an overload

template<class A>
inline std::ostream& operator<<(std::ostream&, const A&)

for a custom type A derived from std::array<T,N>, this leads to ambiguity.

If a custom class S that does not represent a character stream wants to use the s << t syntax in anoter context, this cannot be done for cases where t is an std::array. Furtermore the overload that we provide here may in general not make any sense in this context.

BTW: One may argue that defining overloads for std:: types is in general a very bad idea. Imagine that another library does the same - then we cannot use it together with dune, because both claim to be the authority over std::.