Initial volumetric mesh tracking on tets

[pshriwise]

This contains an initial implementation of volumetric mesh tracking on linear tets. Embree's BVH is currently used for point location purposes. More information on this can be found in #30.

TODOs:

• Implement a MOAB direct access lookup for tetrahedron coordinates
• Add direct access lookup for element adjacencies (if needed)
• Add an interface-level method to determine given element's domain ID
  • + tests
• Separate volumetric point location into its own class (so different implementations can be used)
• Evaluate performance of equivalent MOAB and libMesh models

[pshriwise]

This PR has become too large to reasonably ask someone to review so I'm going to break it up into smaller PRs. This is not an exhaustive list but some PRs will include.

• Add linalg as a new vendored dependency (part of Add tetrahedron containment methods #115)
• Add OpenMP and wrapped interface to support setting threads for libMesh (not entirely happy with this situation)
• new BoundingBox methods, fmt::formatter, and tests (Adding new methods to the BoundingBox class and more tests #114 )
• Vec3da fmt::formatter (Adding an fmt::formatter for the Vec3da type #113)
• Faster next_sidepair_id (Faster/simpler sidepair IDs for libMesh #116)
• MOAB element adjacencies via direct access
• Addition of tet containment method and tests (Add tetrahedron containment methods #115)
• Addition of Embree intersector functions (branch: https://github.com/pshriwise/xdg/tree/embree-tet-funcs)
• Addition of RTCElementRay (this will come with Embree intersector functions)
• Addition of Embree trees for tet elements ( and VolumeElementsUserData)
• Reporting precompiler definitions (maybe better replaced with a clever logger)
• Addition of next_element methods for mesh managers
• Addition of MeshManagerInterface::walk_elements
• Exposure of XDG::segements and XDG::next_element
• Addition of walk_elements mini-app
• Addition of tally_segments mini-app
• Addition of timers for mini-apps

I'll track them here. This list may change as I add or remove them.

In the meantime I can use this branch to start design of related classes in OpenMC.