Pivot table - support groups fields

**Do you want to request a *feature* or report a *bug*?**
- [x] Bug
- [ ] Feature
- [ ] Question

**Did you test against the latest CI build?**
- [ ] Yes
- [x] No

If you answered `No`, please test with [the latest development build](https://ci.appveyor.com/project/ClosedXML/ClosedXML/branch/develop/artifacts) first.

I cloned and ran locally via ClosedXML.Sandbox.

**Version of ClosedXML**

0.102.0

**What is the current behavior?**

Exception during load of a vendor provided spreadsheet.
```
System.ArgumentOutOfRangeException: The column 'xyz' does not appear in the source range. (Parameter 'sourceName')
at ClosedXML.Excel.XLPivotFields.Add(String sourceName, String customName)
at ClosedXML.Excel.XLPivotFields.Add(String sourceName)
at ClosedXML.Excel.XLWorkbook.LoadSpreadsheetDocument(SpreadsheetDocument dSpreadsheet)
at ClosedXML.Excel.XLWorkbook.LoadSheets(Stream stream)
at ClosedXML.Excel.XLWorkbook.Load(Stream stream)
at ClosedXML.Excel.XLWorkbook..ctor(Stream stream, LoadOptions loadOptions)
at ClosedXML.Excel.XLWorkbook..ctor(Stream stream)
...
```

**What is the expected behavior or new feature?**

Spreadsheet is loaded.

**Is this a regression from the previous version?**

Yes. Works in 0.101.0

## Reproducibility

**Code to reproduce problem:**
The exception happens during the load of an existing file with a lot of pivot tables and connections. I'm sorry but a code sample isnt really helpful here.
```
new XLWorkbook("file")
```

- [ ] I attached a sample spreadsheet. (You can drag files on to this issue)

I dont have enough Excel experience to recreate this issue myself and I cant attach the file from the vendor.

---

I did spend a bit troubleshooting in the code and noticed a couple things.

At the previous commit, there was a check that the field is available before trying to add it in several places:

https://github.com/ClosedXML/ClosedXML/blob/69e37ad5f9d936c67d259e46582374930a23dd5f/ClosedXML/Excel/XLWorkbook_Load.cs#L714-L720

I assume this is what avoided the exception that is currently being thrown in `XLPivotFields.Add`.

---

When i skipped the bad adds in troubleshooting, I also ran into an exception where wss is null in ParsePivotSourceReference:

https://github.com/ClosedXML/ClosedXML/blob/9cded9044050523f87567271d72b382505ec2c2e/ClosedXML/Excel/XLWorkbook_Load.cs#L913-L920

`wss` can be null with external sources so it needs to return early.

Start in ClosedXML/Excel/XLWorkbook_Load.cs at the pivot-field loading code around the referenced lines, then inspect XLPivotFields.Add and ParsePivotSourceReference. Reproduce with new XLWorkbook("file") where possible; done means workbooks with missing pivot fields or external sources load without these exceptions, though the vendor file is unavailable.

Written by the indexing model from the issue text.