feat: Add natural convection BC

[damiano-flex]

Greptile Summary

This PR introduces a new NaturalConvectionVerticalSpec class to enhance thermal boundary condition modeling in the Tidy3D framework. The class enables automatic calculation of heat transfer coefficients for natural convection from vertical plates based on fluid properties rather than requiring manual coefficient specification.

The implementation follows standard heat transfer engineering practices by using Rayleigh number calculations and appropriate Nusselt number correlations to determine heat transfer coefficients. It supports both laminar and turbulent flow regimes based on critical Rayleigh number thresholds. The class accepts comprehensive fluid property inputs including thermal conductivity, viscosity, density, specific heat, and thermal expansion coefficient.

The integration is achieved by extending the existing ConvectionBC.transfer_coeff field to accept either a simple float (existing behavior) or the new NaturalConvectionVerticalSpec object, maintaining backward compatibility. The class is properly exported through the main package interface and integrated into the TCAD types system, following established patterns in the codebase for thermal boundary conditions.

Confidence score: 2/5

• This PR has significant issues that could cause problems if merged, particularly around unit consistency and incomplete implementation
• The unit system mismatch between SI units and Tidy3D's micrometer-based system could lead to incorrect calculations, and the _compute_parameters method returns unused values suggesting incomplete integration
• Files tidy3d/components/tcad/boundary/heat.py needs immediate attention for unit consistency, parameter validation, and completion of the computation logic