Decide whether RAII wrappers are implicitly convertible to pointers to the wrapped type

[szhorvat]

Currently, wrappers are convertible to pointers to the wrapped type. An igRealVec can be passed directly where an igraph_vector_t * is expected. This is very convenient, but may have difficult to predict consequences. One particular pitfall that came up was the push_back() member function of typed lists, which needs to behave differently when passed a wrapper of a raw igraph_vector_t *.

igraph-cpp/typed_list_pmt.hpp

Lines 86 to 101 in f2e02ba

	// List takes ownership of t
	void push_back(value_type &t) {
	igCheck(FUNCTION(push_back)(ptr, t));
	t.ptr = FUNCTION(tail_ptr)(ptr); // set as alias
	}
	// List takes ownership of t
	void push_back(value_type &&t) {
	igCheck(FUNCTION(push_back)(ptr, t));
	t.ptr = nullptr;
	}
	// List takes ownership of t
	void push_back(value_type::igraph_type *t) {
	igCheck(FUNCTION(push_back)(ptr, t));
	}

This is because it must transfer ownership from the igVec to the list, which is impossible once the igVec was converted to an igraph_vector_t *. This was solvable by also providing push_back(igVec &&), but it's easy to not notice this situation and make a mistake.

---

An alternative to implicit conversion is to overload the & operator, so that instead of

igRealVec v;
igraph_vector_scale(v, 2);

we would need to write

igRealVec v;
igraph_vector_scale(&v, 2);

This is still very concise.