Feat/refactor model pairing

[phisad]

As discussed in https://github.com/clp-research/clembench/issues/34

• now clem automatically assumes self-play when only a single model is given (and the game is multi-player)
• the cli options are now more verbose (python3 scripts/cli.py run -g -m )
• pipeline scripts have been adjusted (no changes in the games)
• updated README
• removed the option to run all games (as this simplifies the code and is now done in a pipeline script)