Modify level attribute of player as Weak and lazy
Make Level inherit Model

Also some changes to docs.
Clean up world classes

The world ticks now. DummyWorld is a test class. Note that it mixes factory and
operational logic so far though.
Delete publish/subscribe module

It is not required any more, since the networking is about to change.
Add server backend to the loader

The new backend serves as a read-only interface to a given server. The server
has also been altered to support this backend and generally to adapt to the new
server-client communication model.
Add a load_all method to the loader

This method loads all currently active data from the backend to ensure, that
everything is up-to-date in case a backend changes its data on its own, such as
a server backend.
Revise client side logic

The client has been rewritten to actually support the new server-client
communication model. Also a factory function has been written to create a world
instance that locally synchronizes a server via the server backend.

Furthermore, the single instance script has been adapted to run the server
client model. It periodically displays the level randomly modified by the
server.
Move world attribute from Server to LocalServer

Whether a server actually has a world instance or not is an implementation
detail. A remote server, for instance, will communicate with a network resource
instead of maintaining a separate instance.
Update version of weak_and_lazy
Abstract world simulation capability

Not all servers actually simulate a world. But several do, so an intermediate
level of inheritance enhances code reuse.
Add primitive dedicated server

The class DedicatedServer has been implemented subclassing LocalServer, which
itself again replaces WorldServer, because that one was a mistake. Also a
simple connection handler has been implemented.
Add an example echo client controller