Fix WolframModelPlot for Wolfram Language 12.2 (#433)

## Changes
* `ConvexHullMesh` is broken in Wolfram Language 12.2. It seems that only 1D examples evaluate, 2D and 3D example return unevaluated even if using examples from the documentation.
* Because of this, `WolframModelPlot` is currently broken.
* This PR changes `WolframModelPlot` to use the new `convexHullPolygon` (#432) instead of `ConvexHullMesh`.

## Comments
* The tests will fail until #432 is merged.
* I will also update the CI soon to version 12.2 so that the tests on the current master will start failing as well.
* Visually verified `WolframModelPlot` and `RulePlot` on all examples from documentation.

## Examples
* `WolframModelPlot` now works as expected:
```wl
In[] := $Version
Out[] = "12.2.0 for Mac OS X x86 (64-bit) (October 3, 2020)"
```
```wl
In[] := WolframModelPlot[{{1, 2, 3}, {3, 4, 5}, {5, 6, 7}}]
```
![image](https://user-images.githubusercontent.com/1479325/95257565-1e7a3800-07ea-11eb-8325-0f7e269444d2.png)

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Kernel/WolframModelPlot.m

@@ -467,9 +467,7 @@
 
 addConvexPolygons[edgeType_][edge_, subgraphsShapes_] := Module[{points, region, convexPolygons, polygon},
   points = Flatten[#, 2] & @ (subgraphsShapes /. {Line[pts_] :> {pts}, Point[pts_] :> {{pts}}});
-  region = ConvexHullMesh[Map[# + RandomReal[1.*^-10] &, points, {2}]];
-  convexPolygons = Polygon /@ Table[MeshCoordinates[region][[polygon]], {polygon, MeshCells[region, 2][[All, 1]]}];
-  edge -> Join[subgraphsShapes, convexPolygons]
+  edge -> If[Length[points] > 2, Append[subgraphsShapes, convexHullPolygon[points]], subgraphsShapes]
 ]
 
 (** Drawing **)