Feature: Add a scalar and vector data field to VTK write test [2/3]

[holke]

Describe your changes here:

The first step towards #1487.
The vtk_writer example did not write any user data.
Extended the example with 2 data fields, one scalar, one vector and dummy data.
The data fields do already contain values for the ghost elements.

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[holke]

Note: even though writing of ghosts was set true (and not changed by this commit), the ghost elements do not seem to be written, which is odd.

[holke]

Note: even though writing of ghosts was set true (and not changed by this commit), the ghost elements do not seem to be written, which is odd.

Could be resolved, the forest in the example was build without a ghost layer.

[holke]

The tests fail in VTK API mode due to a ghost tree being requested by grid_element_shape and the function expecting only local trees.
@Davknapp is working on a solution.

[Davknapp]

Suggested change

	{
	{
	scalar_data.resize (cells_to_write_count);
	vector_data.resize (3 * cells_to_write_count);

I think it is better to use a single allocation instead of cells_to_write_count many

[Davknapp]

Suggested change

	for (t8_locidx_t icell = 0; icell < cells_to_write_count; ++icell) {
	std::iota(scalar_data.begin(), scalar_data.end(), 0);
	std::transform(scalar_data.begin(), scalar_data.end(), scalar_data.begin(), [](double val) { return val / 10.; });
	std::generate(vector_data.begin(), vector_data.end(), [n = 0]() mutable {
	double scalar_value = (n / 10.);
	double vector_values[3] = { scalar_value, -scalar_value, 42. };
	return vector_values[n++ % 3];
	});

we could replace the loop by stl commands then. It has the drawback of looping three times over the arrray, though, so I would be okay with just removing the allocation from the loop too.