Add contrast limiting adaptive histogram equalization (CLAHE) operator (#2726)

src/operations.h

@@ -35,6 +35,11 @@ namespace sharp {
    */
   VImage Normalise(VImage image);
 
+  /*
+   * Contrast limiting adapative histogram equalization (CLAHE)
+   */
+  VImage Clahe(VImage image, int const width, int const height, int const maxSlope);
+
   /*
    * Gamma encoding/decoding
    */