Current continuous reprocessing comparison with SaltProc uses only a single Serpent2 run

[LukeSeifert]

Summary

The current continuous reprocessing comparison used to compare against SaltProc's batchwise process (which uses 3 day steps) in the MSBR uses a single 3000 day step. This provides results that are not entirely accurate, and comparison with different numbers of steps would also be a useful visualization tool.

Solution

PR needed that allows for any N-steps to be used for Serpent2 continuous reprocessing input. Only requires minor changes to the current code for rerunning and reading previous depletion data.