Support HEX colors for GeneProduct, Regulator, and Reporter

[Gonza10V]

Motivation

• Gene products currently documented with named colors only; callers need to supply HEX color strings (e.g. #00FF00) for plotting and visualization.
• Ensure Regulator and Reporter preserve and propagate caller-provided color values to the base GeneProduct for consistent drawing behavior.

Description

• Documented a color parameter in src/loica/geneproduct.py describing Matplotlib-compatible values including HEX strings.
• Updated Regulator.__init__ to forward color to the base GeneProduct initializer via keyword arguments (sbol_comp and color).
• Updated Reporter.__init__ to forward color to the base initializer and removed a redundant self.color assignment so the base class owns the value.
• Fixed a docstring escape issue by renaming type\_ to type_ and added unit tests in tests/test_geneproduct.py verifying HEX color preservation for GeneProduct, Regulator, and Reporter.

Testing

• Ran pytest tests/test_geneproduct.py tests/test_sbol_export.py which completed successfully.
• All automated tests passed: 10 passed (test suite run shown as 10 passed in 2.40s).

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Codex Task