feat: add RadialKDTreeInterpolatorFactory

[bogovicj]

• and RadialKDTreeInterpolator

This PR adds an interpolator for KDTrees that takes a custom function of the (squared) radius to produce
a RealRandomAccessible. This is code I've used for some years and thought I'd finally push somewhere more common.

// the parameters to build a RadialKDTreeInterpolatorFactory
double maxRadius;
DoubleUnaryOperator squaredRadiusFunction;

RadialKDTreeInterpolatorFactory<FloatType> interp = new RadialKDTreeInterpolatorFactory<>(squaredRadiusFunction, maxRadius, new FloatType());

KDTree tree;
RealRandomAccessible<FloatType> image = Views.interpolate(tree, interp);

The full working demo below produces this output:

// generate points
int N = 12; // number of points
final double bigCircleRadius = 10;
final double angleDelta = 2 * Math.PI / N;
final List<RealPoint> pts = DoubleStream.iterate(0.0, x -> x + angleDelta).limit(N).mapToObj(tht -> {
	return new RealPoint(
			bigCircleRadius * Math.cos(tht),
			bigCircleRadius * Math.sin(tht));
}).collect(Collectors.toList());	

// generate values
final FloatType one = new FloatType(1f);
final ArrayList<FloatType> vals = new ArrayList<>();
IntStream.range(0, pts.size()).forEach( x -> { vals.add(one); });

// build a tree
final KDTree<FloatType> tree = new KDTree<>(vals, pts);

// a function of the squared radius for each rendered point
final double value = 2.0;
final double maxRadius = 2.0;
final double maxSqrRadius = maxRadius*maxRadius;
final DoubleUnaryOperator squaredRadiusFunction = r2 -> value * (1 - (r2 / maxSqrRadius));

// create an image from the points and the radial function
final RealRandomAccessible<FloatType> image = Views.interpolate(
		tree, 
		new RadialKDTreeInterpolatorFactory<>(squaredRadiusFunction, maxRadius, new FloatType()));

// show the image
BdvStackSource<FloatType> bdv = BdvFunctions.show( 
		image, 
		Intervals.createMinSize(-20,-20, 40, 40),
		"rendered points",
		BdvOptions.options().is2D());

bdv.setDisplayRange(0, 2);

[bogovicj]

talked to @axtimwalde - this make make more sense in imglib2 core. I'll close this and move it there shortly