Make the base objects of Crappy children of abc.ABC

[WeisLeDocto]

In the very early days of Crappy, some methods were decorated with abc.abstractmethod to clearly signal that they were meant to be overwritten in subclasses. Since then, the decorators were removed (e.g. a72afa1) and it is now possible to instantiate the base classes or their children without redefining the main methods.

It would be nice to prevent the instantiation of the base classes, as they are anyway not usable as is to achieve anything. Their main methods (like the loop method of Block) should also be protected with an abc.abstractmethod decorator, to make sure they are redefined in children classes.

The target objects to refactor will be the Block, Actuator, Camera, InOut, Modifier, Path, CameraProcess, and Overlay. These classes will inherit from abc.ABC, and define at least one abc.abstractmethod.