Unit test coverage improved

- Unzip shared string table to system temporary file when large inner XML, reduce memory usage about 70%
- Remove unnecessary exported variable `XMLHeader`, we can using `encoding/xml` package's `xml.Header` instead of it
- Using constant instead of inline text for default XML path
- Rename exported option field `WorksheetUnzipMemLimit` to `UnzipXMLSizeLimit`
- Unit test and documentation updated

ref #65, new formula function: INDEX (array formula not support yet)

Keep track of operators per function

Support check string equality with the string value of a defined name

Signed-off-by: NDokiys <dokiychang@gmail.com>

ref #65: new formula functions DURATION and MDURATION
fix incorrect example in SetSheetStyle docs

Improve unit test coverage for the functions: `NewStyle`, `SetActiveSheet`, `SearchSheet` and `deleteAndAdjustDefinedNames`
Simplify code and add comments for the function: `deleteAndAdjustDefinedNames`

Signed-off-by: NMichael Wiesenbauer <michael.wiesenbauer@ambos.io>
Co-authored-by: NMichael Wiesenbauer <michael.wiesenbauer@fau.de>

Fix call getNumFmtID with builtInNumFmt return -1

- fix COUPPCD result accuracy issue
- update close spreadsheet example in documentation and README

typo fixed and simplify code for read the data values arguments of formula functions