Conversion of Gradient-Richardson-Number to stability string. Non-finite
Richardson numbers return NA. This is a diagnostic classification,
not a heat-flux closure check.
The class thresholds follow the current package implementation based on
Bendix (2004) stability-background material.
Usage
turb_flux_stability(...)
# Default S3 method
turb_flux_stability(grad_rich_no, ...)
# S3 method for class 'weather_station'
turb_flux_stability(weather_station, ...)