Go 1.15 and later required, #65 fn: IMABS, IMCOS, IMCOSH, IMCOT, IMCSC, IMCSCH, IMEXP, IMLN and IMLOG10

typo fixed

Default row height compatibility with Apache OpenOffice and Kingsoft WPS, unit test update and typo fixed

workBookRelsWriter, drawingRelsWriter into relsWriter;
drawingRelsReader, workbookRelsReader, workSheetRelsReader into relsReader;
addDrawingRelationships, addSheetRelationships into addRels

- Testing case for set and get doc properties
- Update charts struct XML tags

- go test and godoc has been updated

- go test and document has been updated.

- Support insert new lines into shape, relate issue #38, note that the format set parameter of function `AddShape()` changed;
- go test and go doc updated

- Add a donation link, related issue #57  ^_^

- Drawing `nvPicPr` element ID property calculation changed;
- go test updated

- go test updated

- Doc for function `GetRows()` updated, relate issue #43;
- Fix doc typo in `xmlContentTypes.go`;
- Default template updated;
- Readme updated;
- go test updated