Introducing LinkedList structure for Pie-object

- LinkedList implementation in Pie to manage slices
- new internal-Class "Slice" in Pie
- Pie-objects are now able to add a new slice to themselfs
Adding Methods for Navigating through the Slices of the pie

- Slices now handle there internal id themselves. Important is to always
insert slices after the currentSlice

- next()
-previous()
- first()
- last()
- hasNext()
- hasPrevious()
Adding Basic Iterator
Introducing Pie.update()

- Method to let the pie update its Slices
- for now it checks if the ids of the Slices are in place (counting from
1 up for each Slice)
- opens possibility for live-changing the pie
Introducing getSlice(), isSliceInPie(Slice), findId()

- Methods to select a specific Slice or to check if a Slice is in the
Pie
Introducing removeSlice(Int)

- removes SLice from the pie and lets the following slices close the
gap. Id gets updated and percentage values of the remaining slices as
well