Request for adding API support for Composite charts.

Currently GemBox.Spreadsheet supports Composite charts (charts with mixed data series type) only through preservation.
This means that these elements are preserved so we don't lose any relevant workbook information when loading and saving to an XLSX format, but they are not directly supported by GemBox.Spreadsheet API.

The following link is a demonstration sample of preserving unsupported Chart types when reading from XLSX file and writing to XLSX file:
https://www.gemboxsoftware.com/spreadsheet/examples/excel-chart-preservation/303
More information about GemBox.Spreadsheet's preservation can be found on the following link:
https://www.gemboxsoftware.com/spreadsheet/docs/preservation.html