Remove `AbstractMarkovEnv` and `AbstractZeroSumEnv`

[zsunberg]

After discussion in #40 , I think there is good reason to get rid of AbstractMarkovEnv and AbstractZeroSumEnv. I think we will be able to eventually handle multiplayer environments with a single AbstractEnv. Expect more discussion in #36 .

[zsunberg]

As a consequence of this, player will become an optional interface function

[jonathan-laurent]

What is the standard idiom now to know whether an environment is a one-player environment, a zero-sum environment or a multi-player environment?

[zsunberg]

Hey @jonathan-laurent glad to hear from you :). Currently there is none because I don't have the experience necessary to design one. I will submit a PR with my suggestion shortly, but I am hoping that we can work together to come up with something good!