Pass some SmartPointer variables by reference

[kronbichler]

... instead of by value. I observed in a profiler a lot of constructor and destructor calls to SmartPointer. Since those are not entirely trivial,

dealii/include/deal.II/base/smartpointer.h

Lines 282 to 291 in c76154b

	template <typename T, typename P>
	template <class Q>
	inline SmartPointer<T, P>::SmartPointer(const SmartPointer<T, Q> &tt)
	: t(tt.t)
	, id(tt.id)
	, pointed_to_object_is_alive(false)
	{
	if (tt.pointed_to_object_is_alive && t != nullptr)
	t->subscribe(&pointed_to_object_is_alive, id);
	}

which then calls

dealii/source/base/subscriptor.cc

Lines 135 to 149 in c76154b

	Subscriptor::subscribe(std::atomic<bool> *const validity,
	const std::string &id) const
	{
	std::lock_guard<std::mutex> lock(mutex);
	if (object_info == nullptr)
	object_info = &typeid(*this);
	++counter;
	const std::string &name = id.empty() ? unknown_subscriber : id;
	++counter_map[name];
	*validity = true;
	validity_pointers.push_back(validity);

we should get rid of them.