DataSet valueFormatter does not work.

DataSet valueFormatter does not work.
When I create a DataSet and specify my value formatter it gets ignored and seems like a new DefaultValueFormatter is used.

```
let dataSet = BarChartDataSet(entries: entries, label: "Amount")
dataSet.colors = [.categoryChart]
dataSet.valueFont = .systemFont(ofSize: 11)
dataSet.valueTextColor = .text

// This never renders the correctly formatted values
dataSet.valueFormatter = DefaultValueFormatter(formatter: Formatters.numberWithSeparatorsRounded)

// if I use this blocked based formatter, it's never invoked
dataSet.valueFormatter = DefaultValueFormatter(block: { (value, entry, index, viewPortHandler) -> String in
// this is never called
return "xxx"
})
```

Start by reproducing the issue with the BarChartDataSet snippet and trace how DataSet.valueFormatter is consumed during value rendering. Confirm that both the DefaultValueFormatter instance and its block are invoked, and that their returned text is displayed; no specific file or test is named in the issue.

Written by the indexing model from the issue text.