Initial wall model implementation

[timofeymukha]

Cherry-pick from stress_formulation.

Implements the standard json-based factory pattern for wall models.

• A BAC for wall models.
• A factory routine.
• Two wall models: Spalding's law and a rough log-law.

The wall model sampling is index-based, waiting for better times with more robust probes. The same index is used for the entire wall.