Polyhedron cells for VTKWriter.

This feature implements the support for polyhedrons in VTKWriter. This MR depends on !103 (merged) .

Added 25 commits:

• 16e5449f...7b79d628 - 24 commits from branch master
• 08a0d256 - Merge branch 'master' into feature/vtkwriter-polyhedron

Compare with previous version

Added 7 commits:

• 08a0d256...ecdc1a43 - 6 commits from branch master
• cca37f04 - Merge branch 'master' into feature/vtkwriter-polyhedron

Compare with previous version

Enabled an automatic merge when the build for cca37f04 succeeds

Canceled the automatic merge

Added 1 commit:

• bf480bb8 - [cleanup][VTKWriter] remove unused variable.

Compare with previous version

Enabled an automatic merge when the build for bf480bb8 succeeds

Mentioned in commit f6fd64ef

Status changed to merged

Mentioned in commit c357ac6d

Mentioned in merge request !124 (merged)

Mentioned in commit 8ff4e261

Mentioned in commit 25c4872c