SectorCache::ClearCache to completely clear cache.
Do not assign factions to sector when called from Faction code.
Avoid instantiating Sector directly for calculating distances.
Make sure there is only ever one Sector object for each sector.
Move exploration state to Sector::System
Allow unexplored systems to become explored in game.
Persist explored state of a system.
Lua support for explored systems.
Automatically explore system when entering it.