[ENH] remove private methods from parameters of ProximityForest, `P…

…roximityTree`, and `ProximityStump` (#6046)

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Fixes #5042

#### What does this implement/fix? Explain your changes.
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Removed methods that were being instantiated with the objects of
`ProximityForest`, `ProximityTree`, and `ProximityStump` classes and
made them the class methods.

For the `get_gain` method which defaults to `gini_gain` I added a
dictionary to track the gain functions in case there is more gain
functions added in the future.

Methods changed

- [x] `setup_distance_measure`
- [x]  `get_exemplars`
- [x]  `get_distance_measure`
- [x]  `find_stump`
- [x]  `get_gain`

.all-contributorsrc

@@ -2644,6 +2644,16 @@
       "contributions": [
         "maintenance"
       ]
+    },
+    {
+      "login": "fnhirwa",
+      "name": "Felix Hirwa Nshuti",
+      "avatar_url": "https://avatars.githubusercontent.com/u/67042527?s=64&v=4",
+      "profile": "https://github.com/fnhirwa",
+      "contributions": [
+        "code",
+        "maintenance"
+      ]
     }
   ]
 }