engine update

[mingzhang96]

• 精简的action space，仅保留move, eject, split三个动作
• 更真实的引擎，使碰撞相关的计算更方便，并可以总结出一套可量化的移动规则
• 更快的环境step速度，从整体结构上进行优化
• 新模式：为每个玩家的每个分身球提供单独的动作
• 文档更新

[codecov]

Codecov Report

Merging #41 (5813403) into v0.2.0 (297bb44) will increase coverage by 1.60%.
The diff coverage is 83.29%.

@@            Coverage Diff             @@
##           v0.2.0      #41      +/-   ##
==========================================
+ Coverage   87.69%   89.30%   +1.60%     
==========================================
  Files          63       72       +9     
  Lines        4030     3907     -123     
==========================================
- Hits         3534     3489      -45     
+ Misses        496      418      -78     

Flag	Coverage Δ
unittests	89.30% <83.29%> (+1.60%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gobigger/utils/frame_utils.py	0.00% <0.00%> (ø)
setup.py	0.00% <ø> (ø)
gobigger/managers/spore_manager.py	66.66% <30.76%> (+3.76%)	⬆️
gobigger/envs/__init__.py	36.36% <33.33%> (-63.64%)	⬇️
gobigger/utils/playback_utils.py	41.66% <41.66%> (ø)
gobigger/agents/bot_agent.py	77.84% <50.00%> (-2.28%)	⬇️
gobigger/managers/player_manager.py	73.80% <55.17%> (+0.20%)	⬆️
gobigger/managers/thorns_manager.py	68.91% <60.86%> (-9.46%)	⬇️
gobigger/render/realtime_render.py	67.21% <64.70%> (-28.63%)	⬇️
gobigger/server/server.py	72.54% <67.79%> (+2.48%)	⬆️
... and 59 more

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