Adding mxnet components to rl_coach/architectures (#60)

Adding mxnet components to rl_coach architectures.

- Supports PPO and DQN
- Tested with CartPole_PPO and CarPole_DQN
- Normalizing filters don't work right now (see #49) and are disabled in CartPole_PPO preset
- Checkpointing is disabled for MXNet

rl_coach/architectures/mxnet_components/heads/__init__.py

@@ -0,0 +1,14 @@
+from .head import Head, HeadLoss
+from .q_head import QHead
+from .ppo_head import PPOHead
+from .ppo_v_head import PPOVHead
+from .v_head import VHead
+
+__all__ = [
+    'Head',
+    'HeadLoss',
+    'QHead',
+    'PPOHead',
+    'PPOVHead',
+    'VHead'
+]