Update RigidBodySystem and GUI with external force handling and gravity features

[SaltyJoss]

This pull request makes significant structural and naming improvements to the DSFE_Core codebase, focusing on modernising the CMake build system, refactoring the DSL/interpreter subsystem, and updating telemetry and analysis interfaces for greater consistency and clarity. The most important changes are grouped below:

Build System Modernization:

• Added tinyxml2 as a dependency using CMake's FetchContent and integrated its source directly into the DSFE_Core build, ensuring no external subproject or tests are included.
• Updated the pxmlib dependency to version v1.0.2 in the top-level CMakeLists.txt.

Subsystem Refactoring and Renaming:

• Renamed all Interpreter-related files and namespaces to DSL for improved clarity and future extensibility. This includes files such as Command.h, CommandContext.h, and all command implementations, as well as updating all corresponding includes and namespace usages throughout the codebase. [1] [2] [3] [4] [5] [6] [7] [8] [9]
• Updated the source file organization in CMakeLists.txt to group sources by functional subsystem (SYSTEMS_SRC, LOADING_SRC, PHYSICS_SRC, etc.), and moved DSL/interpreter sources under the src/DSL/ directory. [1] [2]

API and Interface Updates:

• Standardised command result types and method signatures in all DSL command classes, replacing program_data::CmdResult with CmdResult and updating method signatures for consistency. [1] [2] [3] [4]
• Updated the CommandContext API to use more general terminology (e.g., loadRigidBody instead of loadSingleBody/loadMultibody, stopAllVel instead of stopAllOmega, and RigidBodySystem instead of RobotSystem). [1] [2]

Telemetry and Analysis Consistency:

• Changed telemetry and metric logging namespaces from robots to systems and updated method signatures to use RigidBodySystem instead of RobotSystem, ensuring consistency with the rest of the refactor. [1] [2] [3] [4]

Documentation and Metadata:

• Added or updated file-level documentation headers to most header files, specifying the file name, author, and date for improved maintainability and traceability. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

These changes collectively modernise the codebase, clarify subsystem boundaries, and lay the groundwork for future extensibility and maintainability.

DICLAIMER
These changes were aided by AI, quite a lot of what I am doing at the moment is very difficult for me to learn at the same time as correcting and have found that a using it as a DEBUG tool is very helpful. That being said some snippets were derived from provided code.
Additionally, this merge is not void of issues, but allows me to move onto yet another GUI task in preparation for modelling particles and stellar bodies.
I will be revisiting DSL after this next task is complete, specifically looking at adding standard programming language concepts like operators, operations, storage of variables, variable use and assigments, methods, conditionals, and other items like that.
This is so I can actually design REAL modular scripts that mimic a reduced version of what I would do in C++, making it better (not necessarily easier) for others to do the same.
During this stage I will also look into developing the appropriate DSL documentation to aid others (and myself) when writing DSL scripts.