Improve interpolate_gradient description

[Stoeoef]

The current NaturalNeighbor::interpolate_gradient doc comment is a little brief, especially in regard to its example.

An improved version should give a more realistic example with non-constant gradients. Some easy variants would be e.g. interpolating points on a sphere (thus: the normal vector always points away from the origin) or calculating the "average" normal / gradient on the fly by fitting a plane through all adjacent vertices (which might be a useful real-world example!)