Added ability to specify figsize for matplotlib

[jzking]

Plot size variable added to CondensedTree.plot to pass to matplotlib.figsize for convenience.

[lmcinnes]

I think in practice if you specify a particular plot size then you probably want to create a new figure and specify the figsize explicitly for the figure on creation.

[jzking]

My apologies but I am not very well versed in matplotlib nuances. However, I thought specifying plot size in the call to plot made intuitive sense. Perhaps the way I'm implementing it here is not appropriate for general users.