Encode node height distributions in MCC/HIPSTR trees

[rambaut]

Add the information to draw a density for the height distribution for each node in the output tree of treeannotator.

This would allow violin plots to be generated at nodes or densities to be drawn on an axis by tree rendering software (i.e., PearTree - http://github.com/artic-network/peartree).

This could be a vector of densities per height (a histogram), or a shape generated by a KDE, or some controls values for splines or something. The first would be easiest from an implementation standpoint and could be used to create a smooth curve by the visualisation programme.

[msuchard]

or the annotated object could be a string-tag that specifies a file, where the file contains the raw samples or approximated density.

[rambaut]

Possibility - although that then necessitates that the associated files need to be kept associated with the summary tree files (and that all the processing of that is done by the tree drawing program). I was thinking of just a vector of (say) 200 height, density pairs that can be used to draw an approximation to the density curve in a tree rendering package. It would probably only be for nodes >= 0.5 frequency so should be a good enough representation.