Added ggdensitree method

[brj1]

I'm finally ready for a PR. I also merged all my commits into a single commit since the history was messy.

ggdensitree can plot lists of phylogenetic trees (or list of treedata) such that

• tips are aligned in the y axis
• tips can optionally (actually default) be aligned in the x axis
• colouring of edges can be match to an aesthetic/scale

Node and tip labeling can only be done on the first tree in the list. It is slow with a large number of trees (> 100)

[brj1]

Do you think that it would be better to add this functionality to multiPhylo plotting?

[GuangchuangYu]

I think it should works for multiPhylo.

[GuangchuangYu]

I have made some small modifications, mostly for satisfying R check.

And introduce the branch.length = "none" parameter to plot the tree using cladogram by default. Otherwise, it may generate very weird figure using example tree like:

set.seed(2019)
x = ape::rmtree(3, 10)

[GuangchuangYu]

thanks for the PR.