Fix box-car start in DigitalHomodyneSS

Also remove measurement affines.

--CAR
Add fit of cavity line to extract cavity rate \kappa

Hat-tip BRJ for fitting function.

--CAR
API Change - data loading and scaling

loadData -> load_data
-takes optional plot string "real/imag", "amp/phase", "quad"
-returns only data not ampData, phaseData

calScale -> cal_scale
-takes raw data or plot string "top" or "bottom"