Improve checkpoint stats plot

I replaced the boundidx with a "lifetime" - the number of iterations between adding a point to the live pool and when it is removed. This should follow a geometric distribution with scale given by 1 / live.

The two new plots are a trace of the lifetime of each removed point and a histogram with comparison to the theoretical distribution of the lifetimes.

Points at the beginning and end are removed as the statistics take a while to burn-in and don't work for the final live points (marked by different colours on the trace).

I could imagine this being superseded by the insertion index test that nestcheck does (the method implemented here is very similar on some level with the nestcheck test), but I didn't see a simple way to do this.

This is an example of the new figure.