Function strainBridgeCalibration

Builds the linear calibration that turns a strain gauge bridge's amplified output voltage into microstrain.

The bridge output is raw / gain volts, or 1000 * raw / (excitation * gain) mV/V, and the gauge certificate's factor converts mV/V to µε. Subtracting the reading at rest gives:

µε = factor * 1000 * (raw - zero) / (excitation * gain)

so a = factor * 1000 / (excitation * gain) and b = -a * zero.