* Copied over SB3 env checker
* Added test from SB3
* Addition of MIT license attribution and black formatting
* Removed SB3 dependence
* Implemented rough, non-vectorized version of check_nan
* Made some SB3 warnings a bit more "general"
* Removed check for "unsupported" spaces, since Gym should support any space
* Added action/observation checks from pettingzoo, referenced in file docstring
* Removed copy of MIT license in file docstring
* Re-added usage of the check_inf flag for helper functino
* Changed test to be simple/classic example. Should add more tests
* Added API compliance check to registered envs
