Add implementation of mjd_inertiaBoxFluid and Python test

[jiacode777]

Added the function mjd_inertiaBoxFluid to compute the moment of inertia of a rectangular box of fluid (or solid) along its principal axes.

Includes a Python test script (test_inertiaBoxFluid.py) to verify correctness.
Necessary for MuJoCo simulations involving fluid-filled or box-shaped objects to have accurate inertia calculations.

Provides a foundation for eventual C integration in the MuJoCo engine.
how
Inputs: mass, x, y, z (dimensions of the box)

Outputs: Ixx, Iyy, Izz — moments of inertia about X, Y, Z axes.

Formula:

Ixx = mass * (y^2 + z^2) / 12
Iyy = mass * (x^2 + z^2) / 12
Izz = mass * (x^2 + y^2) / 12

Implemented in Python for testing and verification.

Python test script demonstrates correctness; integration into the MuJoCo C codebase can be done once a proper build environment is available.

Compatible with MuJoCo physics conventions and ready for review.

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