Feat/stochastic resolve

[rneher]

This adds a new way to resolve polytomies. The previous polytomy resolution greedily pulled out pairs of child-clades at a time and merged then into a single clade. This often results in atypical caterpillar like subtrees.

This is undesirable since it (i) is very atypical, (ii) causes numerical issues due to repeated convolutions, and (iii) triggers recursion errors during newick export.

The new optional way of resolving replaces a multi-furcation by a randomly generated coalescent tree that backwards in time mutates (all mutations are singletons and need to 'go' before coalescence), and merges lineages. Lineages that remain when time reaches the time of the parent remain as children of the parent.

This new way of resolving is much faster for large polytomies. It is currently implemented as an optional experimental feature that can be used via the flag --stochastic-resolve. Note that the outcome of this stochastic resolution is stochastic!