updated pinned allocator.

// dca::linalg::util::

template <typename T>

class PinnedAllocator : public std::allocator<T> {

class PinnedAllocator {

public:

  T* allocate(std::size_t n) {

  PinnedAllocator() = default;

  using size_type = std::size_t;

  using pointer = T*;

  using const_pointer = const T*;

  using value_type = T;

  T* allocate(std::size_t n, const void* /*hint*/ = nullptr) {

    if (n == 0)

      return nullptr;

    T* ptr;

    }

    ptr = nullptr;

  }

  constexpr bool operator==(const PinnedAllocator<T>& /*other*/) {

    return true;

  }

  constexpr bool operator!=(const PinnedAllocator<T>& /*other*/) {

    return false;

  }

};

}  // util