Include daily summaries in top level metadata

[jeromekelleher]

Store the number of nodes, edges and mutations in the top-level metadata for later analysis.

In principle, keeping the number of nodes could mean that we could recover the state of earlier ARGs without needing to store them all. Because the nodes are added monolithically we should be able to recover the state from a particular date by cutting off the ARG at that time, and then removing any nodes/mutations that were added after that date (by parsimony, etc)