-  added 3 internal function: getSheetID, getActiveSheetID, getSheetNameByID

- Allow empty filter, data, and rows in the pivot table
- Add more test case for pivot table

<sheetData><row r="1"><c r="A1" s="1" t="s"><v>0</v></c><c r="B1" s="1" t="s"></c></row></sheetData>

Remove calculated properties to make recalculate formulas in some spreadsheet applications, such as Kingsoft WPS

Exported function to convert excel date to time

Signed-off-by: Vaibhav Nayak <vaibhav.nayak@gmail.com>

Reduce redundant memory copy in getRowHeight

AddPivotTable API changed: new structure PivotTableField to hold pivot table fields for better scalability

- typo fixed and update do.dev badge in the README.

Resolve #580, revert commit https://github.com/360EntSecGroup-Skylar/excelize/commit/5ca7231ed408ac264f509ff52b5d28ff4fbda757

SetColVisible() can parse a column range + typos.