Multiple colonies on the same world

**What do you want to see?**
When implementing #159, specifically the similarities between ships containing components, colonies containing facilities, and planets containing colonies., the idea of colony space came up. What does colony space mean on a planet? Just hardcode it at 1 for now and make all colonies take up 1 space, but could this be expanded on? What if multiple empires could have colonies on the same world?

**How would it help players?**
Features for modders to play with

**Alternatives**
see #318

**Screenshots and mockups**
N/A

**Things to consider**
How would this even work for players? Would there be ground combat every turn if the empires are hostile? Could this be used in an alternate implementation of ground combat, where both sides have a territory to defend, even the invaders?