add implementation of `mjd_ellipsoidFluid`

[thowell]

mujoco implementation: https://github.com/google-deepmind/mujoco/blob/e4704cd28b2ec37026274b75b66b94082258cacf/src/engine/engine_derivative.c#L1137

the implicitfast integrator depends on this feature to be compatible with the ellipsoid fluid model

[Aaryan-549]

Hey @thowell
I would love to tackle this. I’ve reviewed the C implementation of mjd_ellipsoidFluid linked above, and I’m confident I can port that logic to Warp.

Could you please assign this issue to me?