Aged conditional coalescent

[walterxie]

Conditional coalescent on root age, it may take the PopulationFunction as input.

tree ~ AgedConditionalCoalescent( ... , ParametricDistribution);

1. simulate 1000 trees from PopulationFunction;
2. weight each tree by Pr(tree, age | ParametricDistribution), such as Pr(tree, age | meanlog, sdlog) for LogNormal;
3. resample 1 tree proportional to there weights.