Cleanup imports.

Till now, most of the modules were importing all of the module objects
(variables, classes, functions, other imports) into module namespace,
which potentially could (and was) cause of unintentional use of class or
methods, which was indirect imported.

With this patch, all the star imports were substituted with top-level
module, which provides desired class or function.

Besides, all imports where sorted (where possible) in a way pep8[1]
suggests - first are imports from standard library, than goes third
party imports (like numpy, tensorflow etc) and finally coach modules.
All of those sections are separated by one empty line.

[1] https://www.python.org/dev/peps/pep-0008/#imports

agents/__init__.py

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2017 Intel Corporation 
+# Copyright (c) 2017 Intel Corporation
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -13,26 +13,48 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+from agents.actor_critic_agent import ActorCriticAgent
+from agents.agent import Agent
+from agents.bc_agent import BCAgent
+from agents.bootstrapped_dqn_agent import BootstrappedDQNAgent
+from agents.categorical_dqn_agent import CategoricalDQNAgent
+from agents.clipped_ppo_agent import ClippedPPOAgent
+from agents.ddpg_agent import DDPGAgent
+from agents.ddqn_agent import DDQNAgent
+from agents.dfp_agent import DFPAgent
+from agents.dqn_agent import DQNAgent
+from agents.human_agent import HumanAgent
+from agents.imitation_agent import ImitationAgent
+from agents.mmc_agent import MixedMonteCarloAgent
+from agents.n_step_q_agent import NStepQAgent
+from agents.naf_agent import NAFAgent
+from agents.nec_agent import NECAgent
+from agents.pal_agent import PALAgent
+from agents.policy_gradients_agent import PolicyGradientsAgent
+from agents.policy_optimization_agent import PolicyOptimizationAgent
+from agents.ppo_agent import PPOAgent
+from agents.qr_dqn_agent import QuantileRegressionDQNAgent
+from agents.value_optimization_agent import ValueOptimizationAgent
 
-from agents.actor_critic_agent import *
-from agents.agent import *
-from agents.bc_agent import *
-from agents.bootstrapped_dqn_agent import *
-from agents.clipped_ppo_agent import *
-from agents.ddpg_agent import *
-from agents.ddqn_agent import *
-from agents.dfp_agent import *
-from agents.dqn_agent import *
-from agents.categorical_dqn_agent import *
-from agents.human_agent import *
-from agents.imitation_agent import *
-from agents.mmc_agent import *
-from agents.n_step_q_agent import *
-from agents.naf_agent import *
-from agents.nec_agent import *
-from agents.pal_agent import *
-from agents.policy_gradients_agent import *
-from agents.policy_optimization_agent import *
-from agents.ppo_agent import *
-from agents.value_optimization_agent import *
-from agents.qr_dqn_agent import *
+__all__ = [ActorCriticAgent,
+           Agent,
+           BCAgent,
+           BootstrappedDQNAgent,
+           CategoricalDQNAgent,
+           ClippedPPOAgent,
+           DDPGAgent,
+           DDQNAgent,
+           DFPAgent,
+           DQNAgent,
+           HumanAgent,
+           ImitationAgent,
+           MixedMonteCarloAgent,
+           NAFAgent,
+           NECAgent,
+           NStepQAgent,
+           PALAgent,
+           PPOAgent,
+           PolicyGradientsAgent,
+           PolicyOptimizationAgent,
+           QuantileRegressionDQNAgent,
+           ValueOptimizationAgent]