* Allow to specify trainer callbacks (train_fn, test_fn, stop_fn)
in high-level API, adding the necessary abstractions and pass-on
mechanisms
* Add example atari_dqn_hl
* Add common based class for A2C and PPO agent factories
* Add default for dist_fn parameter, adding corresponding factories
* Add example mujoco_a2c_hl
* Use prefix convention (subclasses have superclass names as prefix) to
facilitate discoverability of relevant classes via IDE autocompletion
* Use dual naming, adding an alternative concise name that omits the
precise OO semantics and retains only the essential part of the name
(which can be more pleasing to users not accustomed to
convoluted OO naming)
* Created mixins for agent factories to reduce code duplication
* Further factorised params & mixins for experiment factories
* Additional parameter abstractions
* Implement high-level MuJoCo TD3 example
