[Feature]: Add a supported Python API

[harryswift01]

Feature Request

Problem / Motivation

SIST is currently centred on command-line execution, which makes it harder to use from scripts, notebooks, and larger scientific workflows.

Users should not need to invoke a subprocess or parse terminal output.

Proposed Solution

Add a documented Python API for configuring, running, and inspecting SIST calculations.

The Python API and CLI should share the same underlying calculation implementation.

Proposed Work

• Add a programmatic configuration object.
• Support loading existing configuration files.
• Add a public calculation entry point.
• Return a supported result object.
• Expose results, metadata, warnings, and execution status.
• Support loading saved results.
• Define clear public exceptions.
• Add API tests and reference documentation.

Done When

• A calculation can be configured and run from Python.
• Invalid configuration raises a clear exception.
• Results can be inspected without parsing terminal output.
• Saved results can be loaded through the API.
• Python and CLI results agree.
• Public interfaces are documented and tested.