chore: improve code readability and style in regressions.py and splitSources.py

[Pronoss]

• Updated docstrings and comments for clarity and grammatical correctness.
• Fixed phrasing: "sources into their own files" → "sources into separate files".
• Improved uncaught exception handling print formatting in splitSources.py.
• Replaced manual string slicing with startswith() for better readability and Python best practices.
• Standardized class naming: regressor → Regressor.
• Improved comment wording for non-ASCII handling.
• Minor whitespace and indentation adjustments.

[github-actions]

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