Add `fitl/cds` an interpolated resampling module with gnuplotter

It's really a pretty trivial application of qtl, but might be useful.

The gnuplot test program is for visualizing a swarm of resamples - an infinite swarm of infinitely thin curves overlaid being roughly the "most accurate" confidence band/error envelope that the inbound bu/edplot point-wise & simultaneous confidence bands "bound" from below & above (if that makes sense to anyone besides myself).