Improvement: redesigns vtk compilation

[faouziH21]

Closes #1537

Describe your changes here:

This PR changes the way vtk compilation is done. Before we used macros to check whether VTK is linked or not and depending on that, some fuctions were defined and some were not. This PR aims to seperate these complications ultimately improving readabilty.

NOTE: not all files have been converted yet. t8_vtk_writer.cxx, t8_vtk_writer.hxx and t8_vtk_writer.h have been omitted to avoid this PR being too large

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[Davknapp]

Can you indent

• ../src/t8_cmesh_vtk_reader.hxx
• ../src/t8_vtk/t8_with_vtk/t8_vtk_reader.cxx
  ?

[Davknapp]

This will be your first contribution to t8code! Congratulations for that. To successfully contribute, can you please add your author file to doc/ . Please name it similar to the others and add a BSD License-statement there. Thank you.

[Davknapp]

Can you address my comment about the documentation? Then we are probably ready to merge.