Proposing to add HATRPO and HAPPO algorithms

[BharathRajM]

Hi I see that most algorithms are in this repository however adding a HATRPO and HAPPO would really help the community benchmark against these two as well. If there's an implementation already available I'd love to build on it, if anyone's open we could collaborate and do a PR

[matteobettini]

Yes this would be interesting an was already mentioned in issue #52 and is part of our open-todos #94

I am happy to review PRs but do not have the bandwidth to implement new features myself