Plot.Clear() doesn't clear everything

This is a good example of a bad software pattern. Switch statements don't get updated when new pottables are added.

This hack works to clear experimental plottables (relates to #249):
```cs
wpfPlot1.plt.GetPlottables().Clear();
```

Perhaps Clear should only have two simple overloads:
* `Clear()` to clear all
* `Clear(Type)` to clear just that plottable type