Add more than one branch to the property plotter

[dstansby]

From #60 (comment):

It would be great to be able to plot more than one cell on the graph, for comparisons. For now, just plotting two sisters (tracks which have the same parent) would be more than enough. Sisters are always members of one single tree, and usually share the track 't' parameter. Here, the colorbar with one blue and one orange track is more than enough!

I was thinking perhaps we could plot all the properties in the whole tree containing the currently selected cell, ie. all the branches being displayed in the main arboretum window.