Remove max from the Kuiper statistic (#1690)

* Remove max from the Kuiper statistic

The reference implementation of uses max() only for the
Kolmogorov-Smirnov statistic; the Kuiper statistic is the difference
between the max and the min.

https://github.com/denismr/incremental-ks/blob/c6bc98833db67d9dc90a0f997a11877860966337/IncrementalKS/Pure%20Python/IKS.py#L55

* Add a changelog entry

Author: e10e3

docs/releases/unreleased.md

@@ -3,3 +3,4 @@
 ## stats
 
 - Added `update_many` method to `stats.PearsonCorr`.
+- Changed the calculation of the Kuiper statistic in `base.KolmogorovSmirnov` to correspond to the reference implementation. The Kuiper statistic uses the difference between the maximum value and the minimum value.

river/stats/kolmogorov_smirnov.py

@@ -270,7 +270,7 @@ def get(self):
         if self.statistic == "ks":
             return max(self.treap.max_value, -self.treap.min_value) / self.n_samples
         elif self.statistic == "kuiper":
-            return max(self.treap.max_value - self.treap.min_value) / self.n_samples
+            return (self.treap.max_value - self.treap.min_value) / self.n_samples
         else:
             raise ValueError(f"Unknown statistic {self.statistic}, expected one of: ks, kuiper")