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

[holke]

Closes #1629

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.

[holke]

This currently fails due ghost trees not being considered in

grid_element_shape<t8_forest_t> (const t8_forest_t grid, const t8_locidx_t itree, const t8_element_t *element)
{
  const t8_eclass_t eclass = t8_forest_get_eclass (grid, itree);

Fix is easy, extend the function to accept ghost tree ids and call "get_ghost_tree_class" if so.
Must still decide whether "ist_ghost" should be added as parameter or will be computed by the function.

Edit: These were already adressed by @Davknapp and should be merged now.