Astrodendro mass-radius plots

[keflavich]

For making the style of plots Jens Kauffmann likes, e.g. Kauffmann 2013 comparing Orion with the Brick. Same as previous pull request onto dendro-core, but this time with the right target.

https://github.com/bradenmacdonald/dendro-core/pull/1#issuecomment-12450895

Thanks for clarifying the split @bradenmacdonald. Might be worth referencing that comment - or copying it - in the docs.

[bradenmacdonald]

Sorry for taking so long to review this. I tested it and it does work, but I think there are a couple changes we should make to the code before merging it.

1. Each Leaf/Branch has an .items property that contains its immediate children, and using that will be much more efficient than using [d for d in item.descendants if d.level==item.level++]
2. Since each python variable for a Leaf or a Branch is just a pointer (behind the scenes), I think it's actually less efficient to have these separate methods that return only specific properties (npix, f_sum), rather than methods that just return a list of matching branches. Of course, the only way to know for sure is to benchmark it, but I feel that the increased code clarity that comes with the latter approach makes it the best way to go.

Here's an example. I think this is working (please test) for the 'brightest' plot mode; I haven't implemented the other version.

def _build_lines(self, items, brightest=True):
        for item in items:

            color = self._color_lambda(item)

            if brightest:
                ### Changes start here ########
                brightest_children = []
                def add_brightest_children(current_item):
                    brightest_children.append(current_item)
                    if type(current_item)==Branch:
                        brightest_child = max(current_item.items, key=lambda i: i.f_sum)
                        add_brightest_children(brightest_child)
                add_brightest_children(item)
                xvalues, yvalues = zip(*[ (i.npix, i.f_sum) for i in brightest_children ])
            else:
                # TODO: normal (non-brightest) mode
            self._plot_values[item.idx] = { 'color': color, 'values': [[xvalues,yvalues],] }

one could also implement this as follows:

            xvalues, yvalues = [],[]
            if brightest:
                def add_brightest_children(current_item):
                    xvalues.append(current_item.npix)
                    yvalues.append(current_item.f_sum)
                    if type(current_item)==Branch:
                        brightest_child = max(current_item.items, key=lambda i: i.f_sum)
                        add_brightest_children(brightest_child)
                add_brightest_children(item)
            else: ...