Bug fixes for Issue #818

[BolunDai0216]

Description

• Changed the action space to [-1, 1]. Internally the action space is [-1, 1] * purser_max_accel.
• Changed the default max velocity and max acceleration of the pursuer to [-0.2, -0.2] and [-0.02, 0.02]
• Changed color of food and poison.
• Made the mass of food smaller by changing its density to 0.01

Fixes # 818

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[jjshoots]

Just curious, what are these magic numbers?

[BolunDai0216]

They should be $\pm\sqrt{2}$, the max speed is defined for $v_{x}$ and $v_y$, so the max magnitude for the speed $v = [v_x, v_y]$ would be $\sqrt{v_x^2 + v_y^2} = \sqrt{2}v_\mathrm{max}$

[jjshoots]

LGTM!

[WillDudley]

lgtm :) thanks for this