LuaTable: remove the ability to Get<LuaTable>(), and implement a Sub() method instead to get a subtable.
LuaTable: Add an alternative Get method with a default value.
Remove the GetVector method and implements iterators instead.

Iterators are a nice and transparent way to be able to have arrays of
tables and fetch them easily, without having an extra interface to
remember.
Port SpaceStation to LuaTable.
SpaceStationType: Rename some variables to be a little more explicit.