Rework factory

[bouthilx]

Why:

The meta-class factory was confusing and sometimes hard to debug. The
factory should be a separate object that the constructors of the
classes. When calling a class, we expect to receive an instance of the
class, not of a child class. The factory also makes it simpler to add
new features such as fetching class objects based on strings on grouping
singleton instances per factory.

How:

Each class requiring the factory will be using Factory(base_class)
as a factory to instantiate child classes. The explicit method
factory.create() will be used instead of the confusing
MyFactory().