post-processing tool

[matsen]

Now that we have serialization down, it will be great to have a post-processing tool.

Two things that will be fun to see:

• average per-generation survival (equivalent to acceptance prob for MH)
• expected MRCA depth (somewhat eqivalent to MCMC autocorrelation function)

For some reason I also think that the average lifetime of a particle would be neat. Note that it's susceptible to edge effects when the generations run out.

We also need a name. @koadman, any clever suggestions?