Add minimal support Pivot tables, Fix #419, Fix #262

[KnyazSh]

The main problem, the solution of which was the purpose of these changes, is to fill excel files with complex graphs and pivot tables using npoi, i.e. using such files as templates. I tested these changes on a relatively large file with several pivot charts, including slicers. I have not checked the creation of the Pivot tables and charts themselves in npoi, although I think it is possible.

Also please pay attention to CT_AlternateContent in [OpenXmlFormats/Vml/SpreadsheetDrawing.cs.](url) I had errors when using innerXml and had to use outerXml. I assume that something is lost by using wrapped innerXml.

[tonyqus]

Before accepting the PR, I suggest you use LoadAndSaveBack tool to test if your change will break the file. Use this tool to load a xlsx file that contains pivot table and see if Excel can still open it after saving back. I used this tool to find a lot of bugs in ooxml implementation.

[KnyazSh]

Before accepting the PR, I suggest you use LoadAndSaveBack tool to test if your change will break the file. Use this tool to load a xlsx file that contains pivot table and see if Excel can still open it after saving back. I used this tool to find a lot of bugs in ooxml implementation.

Thanks for the advice. I already did these tests when I wrote these changes. The main purpose was to remove the error after reading-writing a file.

[tonyqus]

Instead of merging to master, I'd like to create a new branch called Pivot-table.