Generalization of root and tip distributions as RootDistribution and TipDistribution

[kche309]

Allow as optional inputs for TreeLikelihood and GenericTreeLikelihood:

• RootDistribution for specifying root frequencies or a root sequence (for usage with rootfreqs package https://github.com/rbouckaert/rootfreqs)
• TipDistribution for specifying error models on tips (for usage with phylonco package https://github.com/bioDS/beast-phylonco)

RootDistribution and TipDistribution should be abstract classes or interfaces, and packages such as rootfreqs and phylonco should provide the specific models.

These optional inputs should also be handled in TreeLikelihood, BeagleTreeLikelihood and ThreadedTreeLikelihood classes.

In Beauti, when compatible packages are available, it should display additional panels "Root Distribution" and "Tip Distribution", and allow rootfreqs models or error models to be selected from a dropdown.