Cxx: Add OpenVR volume rendering example

See merge request !295

Fix OpenVR cone example (initialize got lost in merge)

See merge request !294

OpenVR: Add note about how to enable interactions

See merge request !293

Use vtkNew in OpenVRCone

See merge request !292

# Conflicts:
#   src/Cxx/GeometricObjects/OpenVRCone.cxx

Fix OpenVR sphere example

See merge request !291

# Conflicts:
#   src/Cxx/GeometricObjects/OpenVRSphere.cxx

Fix OpenVR initialization (fixes crash on VTK >= 9.1)

Closes vtk#18433

See merge request !290

For the interaction to work in the OpenVR examples,
it is required to copy the action/manifest .json files
to the working directory. This adds an according note
to the description of these examples

vtkSmartPointer was used without initialization; changed to probably
intended use of vtkNew

Python code for the example ReadPLOT3D

See merge request !289

Adding opacity

See merge request !288

See: https://discourse.vtk.org/t/shadowlightsdemo-extents-of-shadows-light-frustum-darkened-areas/3410

Fixes identified by Clang-Tidy & Clazy

See merge request !287

minor fixes for chp. 3.7

See merge request !286

Fixing vertex ordering

See merge request !284