replace removed scipy.interpolate.spline

scipy.interpolate.spline was deprecated in 0.19.0 and removed in 1.3.0 see here.

This is used for the calibration plotting. I updated the call to scipy.interpolate.spline to use scipy.interpolate.interp1d. This is the same method as is used in the calibration module. We may want to make a change down the line to make the plotting use the code from the calibration module.

This is currently broken in the current ligo cvmfs builds so we should fix ASAP.