StarSystem Refactor

**Work in Progress**

...so very work in progress.  But I thought I'd better get what I have so far up before I start heading down into the depths of population generation and SystemBody (where everything depends on everything else), so people can at least see whether it compiles on something other than my machine, and get a little feedback on the story so far.

I'm trying to be fairly modest in goals here, and 'just' pull the system generation part of StarSystem and friends out into a separate class, so we can see what the implied interface is and perhaps make it an abstract super-class which would give a place to start if someone wanted to write a new system generator.

Good stuff like Merging Sector::System and CustomSystem with StarSystem will have to wait a bit.

Also, I'm attempting to avoid going down the rabbit hole of improving the generator output.