Create basic economic backend

[BarthPaleologue]

From each StarSystemModel, a StarSystemEconomyModel will be created. It will describe the type of economy: none if not inhabited, regular capitalist, communist and other variants will come with time. It will also describe the abondance of resources in the system (though this will probably its own component in the future).

The player will also be able to use money, different currencies in different regions of space. It will be carried through the save file. In this first basic iteration, money will be used to refuel the tanks of the spaceship. Later it will be used to add components to the spaceship (in a similar fashion to the landing computer).