Add several wrappers to convert automatically between different reductions of TreeTraits (sum over tree, sum across sites, etc.).  One can even double-wrap to get sum over tree + sum across sites.   This should reduce substantial code-duplication and earn me a gold star.
Add several wrappers to convert automatically between different reductions of TreeTraits (sum over tree, sum across sites, etc.).  One can even double-wrap to get sum over tree + sum across sites.   This should reduce substantial code-duplication and earn me a gold star.

benb/beast-mcmc,benb/beast-mcmc,benb/beast-mcmc,benb/beast-mcmc,benb/beast-mcmc