Initial delta tracking implementation

[pshriwise]

Changes with particle tracking updates and new transport loop (relatively recently rebased):

The basics, updated tracking and a naive approach at global majorant cross-section generation (no energy grid modification):
https://github.com/pshriwise/openmc/tree/delta_tracking_rb

Hybrid tracking, switching between surface and delta tracking depending ratio of the current total cross section to majorant cross section based on a threshold parameter.

https://github.com/pshriwise/openmc/tree/delta_tracking_hybrid

Preliminary results for this implementation can be seen on the related project info here

[pshriwise]

A version that I'd feel comfortable opening a PR as an initial cut of this feature relies on the resolution of #3342 in the branches above.