add heater models

[joamatab]

Summary by Sourcery

New Features:

• Adds a simple phase shifter model for a straight heater with metal.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request introduces a new model for straight heaters with metal on top (straight_heater_metal_sc) and its silicon on insulator variant (straight_heater_metal_so). The model calculates transmission based on applied voltage, wavelength, effective index, and other relevant parameters.

No diagrams generated as the changes look simple and do not need a visual representation.

File-Level Changes

Change	Details	Files
Added a new model for straight heaters with metal on top, including both single crystal and silicon on insulator variants.	Implemented straight_heater_metal_sc function to model the heater's behavior based on voltage, wavelength, and other parameters. Defined straight_heater_metal_so as an alias for straight_heater_metal_sc.	cspdk/si220/models.py

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