Enhance sparse_rail_generator

As discussed in the weekly call lets enhance the current sparese_rail_generator. The idea is as follows:

Enahnce single node city modules to multi-node cities.

Idea 1:

Each city has a center node and at each boarder of the city North/East/South/West it has one or ceveral nodes that correspond to connections to other cities. The structure within the city boundaries is either:

• generated through some random process that makes realistic train stations
• or we use templates generated by adrian.

Idea 2: Same as above, but we don't orient the outer city connections accordin to directions but just randomly distirbute them

Level Generation:

1. Distirbute center city nodes and intersection across the map
2. Generate city boarders and add connecting nodes of cities and intersection
3. Connect cities and intersections together
4. generatre inner city structures (maybe also for intersection)
   1. Through random mutli node process
   2. Through templates
5. Place trainstations within cities and store city a -> city b connections for schedule

The image below shows a sketch of the idea.