Engine - support agent nicknames to shorten execution arguments

Currently a common execution line could look like this:
```
splendor -a Engine.agents.our_agents.ppo.ppo_agent,Engine.agents.our_agents.minmax --agent_names=ppo,minimax -t -m 10
```
It would be a whole lot easier & nicer if it would look like this:
```
splendor --nicknames ppo,minmax --agent_names=ppo,minimax -t -m 10
```