Pie Chart entries outside slices have different position offsets

* [ ✔️] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

I was trying to render a pie chart with entries outside slice without value lines. I tried to play with `valueLinePart1Length` and `valueLinePart2Length`, but this just helped a little, didn't solve the problem completely. Also I went through all the variables of `PieChartDataSet`.

## What did you expect to happen?

1. Align the entry text to be at the center outside the slice. [Look at 1 & 3]
2. Keep all the slice texts equidistant from the slice. [Look at 3 & 6]

## What happened instead?

The position offset of entries outside each slice are variable both in distance from the slice as well as in alignment.

## Charts Environment

**Charts version/Branch/Commit Number:** 3.0.4
**Xcode version:** Xcode 9.4.1
**Swift version:** Swift 4.1
**Platform(s) running Charts:** iOS
**macOS version running Xcode:** 10.13.6