#15 manual dispersion fails with too few points

[Christer Sandin]

In the case that the selected lines cover only a portion of the entire range the spline routine may fail horribly and ruin all line positions. This has to be fixed somehow; possibly by setting the rational spline parameter (sigma in SPLINE) to, say 0.5 instead of the default 1.0 (this change is not implemented in the code). Note, for a vast majority of cases this is not a problem.