How a sensor module estimates a surface normal from a patch.

Mirrors tbp/monty/frameworks/models/sensor_modules.py::SurfaceNormalMethod, which is an enum of three names: total least squares (the default Monty's experiments use), ordinary least squares, and a naive cross product of neighbouring pixels.

Only total least squares is ported — it is the one Monty's configurations ask for, and the only one Monty.Frameworks.Utils.SensorProcessing implements. Asking for another is an error rather than a silent substitution: a normal estimated a different way is a different percept.