Unified Interface for Code Cities

[tinxx]

In #753, a process for the synchronization of Code Cities has been implemented. Code Cities are stored on a back-end server and downloaded by clients when the server is joined.

Currently, the instantiation of downloaded Code Cities expects the process to be equivalent to that of a SEECity.

The goal of this issue is to define an interface that can be used instead. This is necessary because not all Code Cities extend SEECity and even if they do, they might implement additional steps for loading and drawing data.

[koschke]

SEECityEvolution inherits from AbstractSEECity, so it makes sense to just add abstract methods to the latter.

Please take note that those cities are just containers for parameterizing the creation of code cities. The actual loading is done by classes derived from GraphProvider, which are part of a code city's parameters.