Data Labels refactor

[OssianEPPlus]

Several things in our handling of data labels on charts should be improved;

1. Setting datalabel.Position can cause corrupt workbooks. It should be limited based on chart-type or throw when set to something that will cause corrupt workbook.
2. Series, XSeries and Data Label Range are essentially the same structure and should be unifed as a class/method for mapping range data to a chart.
3. Series[0].Datalabel.SetRange() does not set the runs or field items in the TextBody xml item. eg. type = EXCELRANGE
4. this should be done to ensure older versions of excel can open the file. It has a very perculiar behaviour when Value, Category and Data Label Range are set.
5. SeriesDataLabel should have easier access to Series
6. DataLabel(s) have A Lot of different entry points both in the xml and in epplus. The overall structure should be more user-friendly if possible. (e.g. Chart.Datalabel, chart.series[0].datalabel, chart.series[0].datalabel.datalabels[0]. As well as EX versions)
7. Verify the entry-points respect the hierarchical rules of specific beats general but that they are initialized to parent values.
8. Verify all charts actually implement data labels if they have them in Excel.