ENH: singlet fitting should return a baseline parameter

[bennyrowland]

Currently singlet fitting will return an FID with the best fit, but this does not incorporate any baseline estimation. As the first few points are not used in the fit as they are assumed to include baseline, this usually means that the fit and the data do not align very well. It would be better if the results included a baseline estimate as well, which could be derived by applying a spline smoothing to the difference between fit and data. Additional parameters should be added to the fit function controlling the smoothness and number of knots on this baseline.