not-a-knot end conditions

[AlexRockliff]

Matlab has a GriddedInterpolant function which when used in "spline" interpolation mode, can generate cubic splines for n dimensional data, like splinter. However, Matlab's GriddedInterpolant function uses not-a-knot end conditions, which gives it distinctly different characteristics to splinter near the edges of the data set.

Support for optionally using not-a-knot end conditions when building a spline would be really useful.