Better interfaces and names for Actor, Critic, Net and other classes

[MischaPanch]

All annotations with nn.Module should be replaced. @opcode81 feel free to extend the description, if you want to.

We could make it backwards compatible and use of the deprecation project by doing something like

@deprecated(
    deprecated_in="1.1.0",
    removed_in="2.0.0",
    details="This class has been renamed. Use CriticDiscrete instead",
)
class Critic(CriticDiscrete):
   pass