[bugfix] Fix gready overload of operator<< for StaticMultiIndex

When overloading the free operator operator<<(const S&, const T&) for a stream S and a specific type T that we have under control, one should not provide a gready overload for template<class S> but use std::ostream. Here we do not want to define what s << StaticMultiIndex() means for any type S but only for cases where we write into a character stream.

A downstream library may want to define the same operator for a type S under its control, e.g. to define its own custom semantics for s<<t.