If cell contains formula then cell's DataType is not properly recognized

## Read and complete the full issue template

Do not randomly delete sections. They are here for a reason.

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

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

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

**Version of ClosedXML**
0.95.999.2073
e.g. 0.95.3

**What is the current behavior?**
If cell contains formula(1+ 2) then cell's DataType property shows as Text even though cell datatype set on UI is Number.

Complete this.

**What is the expected behavior or new feature?**
If cell contains formula(1+ 2) then cell's DataType property should show Number as it is set on UI.

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**Is this a regression from the previous version?**
No

Regressions get higher priority. Test against the latest build of the previous minor version. For example, if you experience a problem on v0.95.3, check whether it the problem occurred in v0.94.2 too.

## Reproducibility
**This is an important section. Read it carefully. Failure to do so will cause a 'RTFM' comment.**

Without a code sample, it is unlikely that your issue will get attention. Don't be lazy. Do the effort and assist the developers to reproduce your problem. Code samples should be [minimal complete and verifiable](https://stackoverflow.com/help/mcve). Sample spreadsheets should be attached whenever applicable. Remove sensitive information.

**Code to reproduce problem:**
```c#
public void Main()
{
string fileName = @"";
var xlworkBook = new XLWorkbook(fileName);
if (xlworkBook != null)
{
var sheet1 = xlworkBook.Worksheet("Sheet1");
if (sheet1 != null)
{
var rows = sheet1.RangeUsed().RowsUsed().Skip(1);
foreach (var row in rows)
{
var rowCell = row.Cell(1);
var cellDataType = rowCell.DataType;

Debug.WriteLine("Cell Position: " + rowCell.Address.ToString() +
", Cell Datatype: " + rowCell.DataType.ToString() +
", Cell Has formula: " + rowCell.HasFormula.ToString());

}
}
}
}
```

- [
[CellDataTypeProblem.xlsx](https://github.com/ClosedXML/ClosedXML/files/5084127/CellDataTypeProblem.xlsx)
x] I attached a sample spreadsheet. (You can drag files on to this issue)

Start with the attached CellDataTypeProblem.xlsx and the XLWorkbook, Worksheet, RangeUsed, Cell, DataType, and HasFormula entry points shown in the reproduction. Trace how a formula cell's data type is loaded, then verify that the formula cell reports the numeric type set in Excel and add a regression test covering the workbook.

Written by the indexing model from the issue text.