Solar system flocking

[dantaeyoung]

• [ ] Implement simple flocking:
  • [ ] Planets fly towards other planets that have similar book-topic-mixtures
  • [x] Planets maintain a distance from other planets
  • [x] Planets maintain a maximum distance from a central point (0,0,0)
• [x] Planet view reflects individual planet
  • [x] change in size based on book checkout #
  • [x] color matches individual planet color