Astrodendro mass-radius plots 

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.