GSL.owner: Ownership pointers

• unique_ptr<T> // unique ownership: std::unique_ptr<T>
• shared_ptr<T> // shared ownership: std::shared_ptr<T> (a counted pointer)
• stack_array<T> // A stack-allocated array. The number of elements are determined at construction and fixed thereafter. The elements are mutable unless T is a const type.
• dyn_array<T> // ??? needed ??? A heap-allocated array. The number of elements are determined at construction and fixed thereafter. The elements are mutable unless T is a const type. Basically a span that allocates and owns its elements.