Reduce split particle memory in secondary banks

[pshriwise]

Description

When we split particles during variance reduction, the source sites generated using Particle::split are identical in nature. To reduce storage in the secondary particle bank, it would be useful to allow source sites in the bank to be "reused" a number of times when sourcing them to reduce memory overhead in this data structure.

Alternatives

If supporting this involves too much additional complexity or we're solving a problem that isn't there (pending some data collection on the size of the secondary arrays), then it might be best to leave it as-is.

Compatibility

This will likely change the SourceSite struct, which has ramifications for the statepoint spec and MPI communication. It will also require some additional bookkeeping.