CUDA path tracer base release

Unidirectional path tracer implemented in templated CUDA C++. Currently, this repo supports:

• Scene parsing from xml file (similar to my AdaPT renderer).
• Shared mem accelerated ray-scene intersection.
• CUDA virtual function based BSDF (Lambertian, Specular and Translucent)
• variant based primitive types
• Simple next event estimation for point emitters and area emitters.

Next step: Incorporate Nvidia OptiX for further accelerated ray tracing.