Merge topic 'enable-ospray-unstructured-volume-rendering'

a39165b8 silence an inconsequential runtime warning in paraview
2aad367e fixup the test and add a baseline image
77b7e381 minor cleanup to get array we want just once and safely
773f3111 Style changes to conform to vtk style guide
37208999 promote the array getter so it can be reused
1758291e back out the unlrelated appearance improvements
aebe2c66 fix header test and use the recent c++11 markups
7d60f881 adding tet mapper
...

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Sankhesh Jhaveri <sankhesh.jhaveri@kitware.com>
Merge-request: !3507