Switch to ray tracing library

[nicolas-f]

Test Embree for SPPS propagation of particles

https://github.com/embree/embree

Apache 2 license is compatible with I-Simpa license

[nicolas-f]

Or use one of theses structures:

• BVH (like embree)
• Octree
• 3d bsp

In order to remove the need of tetgen for SPPS

• Add computation core validation in tests

[nicolas-f]

Embree is not robust (rendering purpose) and should not be used for intersection test in acoustic simulation https://github.com/embree/embree/issues/188 https://github.com/embree/embree/issues/174 https://github.com/embree/embree/issues/181