add density in cfg

[mingzhang96]

说明

config 中添加了 match_ratio 字段。表示比赛setting的配置比例。默认是1.0。这个值将会乘到manager里涉及到球球数量的参数和地图大小上，来保证等比例对场景进行缩放。例如如果这个值设为0.5，则地图宽高会乘以 math.sqrt(0.5) 来保证面积为原来的0.5倍，然后地图内球球的数量会乘以0.5。

使用

server = Server(cfg=dict(match_ratio=1.0))

[codecov]

Codecov Report

Merging #38 (d1ad5f1) into v0.2.0 (184f643) will decrease coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v0.2.0      #38      +/-   ##
==========================================
- Coverage   88.29%   87.73%   -0.56%     
==========================================
  Files          63       63              
  Lines        3974     4021      +47     
==========================================
+ Hits         3509     3528      +19     
- Misses        465      493      +28     

Flag	Coverage Δ
unittests	87.73% <100.00%> (-0.56%)	⬇️

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

Impacted Files	Coverage Δ
gobigger/server/server_default_config.py	100.00% <ø> (ø)
gobigger/server/server.py	68.42% <100.00%> (-3.41%)	⬇️
gobigger/managers/thorns_manager.py	78.37% <0.00%> (-5.41%)	⬇️
gobigger/players/human_player.py	89.33% <0.00%> (-1.34%)	⬇️
gobigger/render/env_render.py	92.97% <0.00%> (-1.29%)	⬇️
gobigger/hyper/actions.py	53.62% <0.00%> (-1.21%)	⬇️
gobigger/balls/spore_ball.py	100.00% <0.00%> (ø)
gobigger/render/test/test_env_render.py	100.00% <0.00%> (ø)

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