Speed Improvement
1. replace dm-tree with treevalue (206, wangsiping97, dmc 6k->8k FPS)
2. move python computation logic into c++ (208, wangsiping97, dmc 8k->11k FPS)
3. cache c++ object read result (209, wangsiping97, dmc 11k->14k FPS)
4. improve gym speedup (210, wangsiping97, gym 11k->14k FPS)
Now we are truly faster than dm_control!
Enhancement
1. upgrade examples to support gym >= 0.26 (215, 51616)
3. update demo links in README (212, Benjamin-eecs)
4. better arugment checking in python side init (217, Trinkle23897)
6. refactor some tests (210, Trinkle23897)
1. fix gym action space seed behavior error (211, Trinkle23897)