Global shared secondary bank

[pshriwise]

For problems employing variance reduction the workload per task becomes highly variable depending on how many splits the particle undergoes. A global secondary bank would allow us to balance this workload by allowing split particles to be added and picked up from any thread. The challenge here is the implementation of an efficient and robust stack for concurrent addition and removal of particle source sites.

[paulromano]

Perhaps we should move this forward in the project plan to support #9 and #10