Compact_container: add an overload s_iterator_to(pointer)

[lrineau]

Note that the UBAN sanitizer does not like that code, for a null cell pointer:

cgal/Poisson_surface_reconstruction_3/include/CGAL/Poisson_reconstruction_function.h

Lines 261 to 264 in 6547f64

	Cell_handle get() const
	{
	return Triangulation_data_structure::Cell_range::s_iterator_to(*m_cell);
	}

Fixed by 9cd4535. Now:

cgal/Poisson_surface_reconstruction_3/include/CGAL/Poisson_reconstruction_function.h

Lines 261 to 267 in 9cd4535

	Cell_handle get() const
	{
	if(m_cell == nullptr)
	return {};
	else
	return Triangulation_data_structure::Cell_range::s_iterator_to(*m_cell);
	}

Maybe s_iterator_to in the compact container should have an overload for pointers, to avoid that extract test for null.

Originally posted by @lrineau in #8248 (comment)