Add: implementation MF-PPO

[rubensolozabal]

MF-PPO algorithm implemented in the paper:

@inproceedings{algumaei2023regularization,
  title={Regularization of the policy updates for stabilizing Mean Field Games},
  author={Algumaei, Talal and Solozabal, Ruben and Alami, Reda and Hacid, Hakim and Debbah, Merouane and Tak{\'a}{\v{c}}, Martin},
  booktitle={Pacific-Asia Conference on Knowledge Discovery and Data Mining},
  pages={361--372},
  year={2023},
  organization={Springer}
}