First locally-run demo

Changelog since last release:

• Habitat drawing is now not wonky (updated the origin of the vpython canvas to be the habitat)
• Habitat should (but doesn't) have an arrow drawn where it's velocity vector is pointing
• Habitat has some textures now
• Habitat has, theoretically, the backend functionality to spin and use its engines. No GUI way of doing that yet.
• Made physics engine more stable, Phobos now does not get ejected from the Mars system (put physics engine in background thread, decreased max simulation interval to scale sub-linearly with the time acceleration, and added a three-body test)

Hi Nova!