Inverse of interpolated model is misleading

[minnerbe]

Currently, the implementation of InterpolatedAffineModel2D creates its 'inverse' by inverting its constituents:

public InterpolatedAffineModel2D<A, B> createInverse() {
	final InterpolatedAffineModel2D<A, B> inverse = new InterpolatedAffineModel2D<>(a.createInverse(), b.createInverse(), lambda);
	inverse.cost = cost;
	return inverse;
}

This leads to model.createInverse().apply(point) not being equal to model.applyInverse(point), which is (correctly) implemented as applying the inverse of the internally computed interpolated AffineModel2D.

I find this behavior to be very misleading to the point that I don't see any value in implementing createInverse() at all.
Rather, I think it should just throw an UnsupportetOperationException, since there is no way to guarantee that the interpolated model is invertible just by having invertible constituents.

[minnerbe]

This is addressed by #69 .