#80 Improve boundary layer models with compressible DeChant skin friction/Van Driest heat flux.

[nlaing613]

This pull request introduces compressible closed-form models for skin friction and wall heat flux and modularizes these models and their inputs.

The main changes are located in:

• models/wall_models.py
  Contains separate heat flux and skin friction classes and wall-state (key parameters used by models).
• models/boundary_layer.py
  Simplified calculation by offloading to wall_models.py.

Included changes

• Explicit wall-model selection via: wall_models=(SkinFriction, HeatFlux)
• Refactored the boundary-layer implementation to use a WallState abstraction (get_wall_state), decoupling wall-model evaluations from the core solver and enabling modular skin-friction and heat-flux models
• Allowed wall temperature specification without requiring a heat-flux model for Cf calculation.
• Made boundary-layer instantiation depend on the selected skin-friction model rather than the previous boolean flag
• Added Van Driest recovery-temperature model for compressible heat-flux calculations
• Retain low-Re analytical solutions incompressible skin friction and heat flux.
• Updated the constant-area duct validation case naming/context

[TimothyEDawson]

@nlaing613 I think this is good to go! All of the tests and example cases look good to me. The results from optimization.py seem to be a little bit different, might be worth digging into that a bit more later.