Ease printing a ddc::ChunkSpan to the standard output

[tpadioleau]

To be used in the examples to replace the following calls

ddc/examples/heat_equation.cpp

Lines 73 to 75 in a983ed3

	ddc::for_each(ddc::get_domain<DDimX>(temp), [=](ddc::DiscreteElement<DDimX> const ix) {
	std::cout << std::setw(6) << temp_slice(ix);
	});