`nearest_n_within` does not limit the number of items when not sorted

[Nahor]

In kiddo v4.2.0, nearest_n_within will only limit the number of items if sorting is enabled. If it's disabled, all the items within the radius are returned.

E.g. in the cities example, replace nearest_n with nearest_n_within and a radius of 10km.

-let nearest_5_idx = kdtree.nearest_n::<SquaredEuclidean>(&query, 5);
+let nearest_5_idx = kdtree.nearest_n_within::<SquaredEuclidean>(
+     &query,
+     kilometres_to_unit_sphere_squared_euclidean(10.0),
+    5,
+    false,
+);

This outputs 13 cities (for my data set) instead of the expected 5. If the list is sorted, then it works as expected.

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Also note that the example in the documentation for nearest_n_within creates a tree of 2 items, asks for 2 items, and then is happy because it got 2 items in return!