[BUGFIX] Fix planes have epsilon mass

[hughperkins]

Description

• added unit test to check that planes do not have mass EPS
• added fix so that planes no longer have EPS mass

Notes:

1. test:
   • PASSes on this PR
   • PASSes on v0.3.11
   • FAILs on main
2. heavily cursored...

to test:

pytest -svx -n 0 tests/test_plane_mass.py

Related Issue

Resolves Genesis-Embodied-AI/Genesis#

Motivation and Context

How Has This Been / Can This Be Tested?

Screenshots (if appropriate):

This branch:

main:

Checklist:

• I read the CONTRIBUTING document.
• I followed the Submitting Code Changes section of CONTRIBUTING document.
• I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• I updated the documentation accordingly or no change is needed.
• I tested my changes and added instructions on how to test it for reviewers.

• I have added tests to cover my changes.
• All new and existing tests passed.

[duburcqa]

Bug fixes should be physically motivated. Your unit test is checking that your implementation is correct, but not that it makes any physical sense. By the way, the mass of objects that are fixed wrt the world is undefined in essence, so computing it asking for it should rather raise an exception.

[github-actions]

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