Feat: Add ClusterMotor class and inertia tools

[ayoubdsp]

Pull request type

• [x] Code changes (bugfix, features)
• [x] Code maintenance (refactoring, formatting, tests)
• [ ] ReadMe, Docs and GitHub updates
• [ ] Other (please describe):

Checklist

• [ ] Tests for the changes have been added (if needed)
• [x] Docs have been reviewed and added / updated
• [x] Lint (black rocketpy/ tests/) has passed locally
• [ ] All tests (pytest tests -m slow --runslow) have passed locally
• [ ] CHANGELOG.md has been updated (if relevant)

Current behavior

Currently, RocketPy only supports attaching a single Motor object to a Rocket. Users wishing to simulate a cluster configuration must manually aggregate thrust curves and mass properties externally before creating a generic motor instance, which is cumbersome and less accurate regarding geometry.

New behavior

This PR introduces the ClusterMotor class in rocketpy/motors/cluster_motor.py.

Key features:

• Aggregation: The class accepts a list of Motor objects, their positions, and optional orientations.
• Mass & Thrust: It automatically computes the total propellant mass, dry mass, and total thrust vector (scalar sum for now) as functions of time.
• Geometry: It computes the global Center of Mass (CoM) and Center of Propellant Mass (CoPM) of the cluster.
• Tools: I have added the necessary mathematical helper functions (Parallel Axis Theorem for moments and products of inertia) in rocketpy/tools.py to support future advanced inertia calculations.

Breaking change

• [ ] Yes
• [x] No

Additional information

Note to reviewers regarding Inertia:

I have already implemented the necessary Parallel Axis Theorem functions in rocketpy/tools.py within this PR. The full integration of these tools into ClusterMotor (to calculate the full inertia tensor dynamically) is planned for a dedicated follow-up PR immediately after this one is merged.

This incremental approach ensures the basic structure is validated before adding mathematical complexity.