Added a head volume renderer, which uses 99 image slices.

Notes:
-Head volume renderer uses the same shaders as the heat map renderer.
-The head data needs to scaled (it's a 1:1:2 scale).

To Do:
-Add sliders
-Add separate shaders
Merge branch 'master' of github.com:schulchr/BVR into head_renderer
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Merge branch 'head_renderer' of github.com:schulchr/BVR into head_renderer
Merge pull request #1 from schulchr/head_renderer

Head renderer