Add log partitioning function

[ashton314]

It would be nice to have some sort of function + utilities to separate the logs into semantically meaningful chunks. Motivation: as a simulation progresses, things can get increasingly unstable. We can diff CSTGs between groups of iterations to watch the instability progress.

Consider adding a function like partition_log("label_name ← (idk if we want any parameters in this function—think about it)"), and maybe some utilities to separate, analyze, and diff the logs/CSTGs.

[bennn]

@ashton314 try a quick version later this month --- for the JuliaCon talk. Partition in the main loop of shallow waters, or something similar.

Don't make a public interface for it yet. First let's see if the idea works at all. (I think it will, but hey this is research.)