Added unique_by function

docs/content/3.manual/manual.yml

@@ -931,6 +931,21 @@ sections:
             input: '[1,2,5,3,5,3,1,3]'
             output: ['[1,2,3,5]']
 
+      - title: `unique_by`
+        body: |
+          
+          The `unique_by(.foo)` function takes as input an array and produces
+          an array of the same elements, in sorted order, with
+          elqements with a duplicate `.foo` field removed. Think of it as making
+          an array by taking one element out of every group produced by
+          `group_by`.
+        
+        examples:
+          - program: 'unique'
+            input: '[1,2,5,3,5,3,1,3]'
+            output: ['[1,2,3,5]']
+
+
       - title: `reverse`
         body: |