Fixes #34

- now clem automatically assumes self-play when only a single model is given (and the game is multi-player)
- removed the option to run all games (as this simplifies the code and is now done in a pipeline script)
- improved cli usage `python3 scripts/cli.py run -g <game> -m <model1> [<model2>]`
- pipeline scripts and README have been updated (no changes in the games)