Remove Discrete Observations

[awjuliani]

• Remove discrete observations as separate category of vector observations. As such, there are now no discrete or continuous vector observations, only vector observations.
• Environments which use discrete observations can now use AddVectorObs(index, range) in order to add one-hot vectors to the observations.
• Modified documentation appropriately.
• Changed Basic environment appropriately, and re-trained model.
• Rebuilt protobuf definitions to reflect this change.
• Upgrade protobuf to 3.6.0.

[xiaomaogy]

If you do a ctrl+shift+f in the whole repo for keyword "discrete", there is still some places that mentions discrete observations.

For example:

In docs/Getting-Started-with-Balance-Ball.md:
The Discrete vector observation space is an index into a table of states.

In docs/Learning-Environment-Design-Agents.md:
For more information on CNNs, see this guide. You can use camera observations and either continuous feature vector or discrete state observations at the same time.

[awjuliani]

Thanks for pointing this out @xiaomaogy. I have made the changes.