* Add funcs to remove sheet from workbook
- RemoveSheet added to remove sheet by index
- RemoveSheetByName added to remove sheet by its name
* Add `CopyRelationship` func to `Relationships`
* Add funcs `CopySheet`, `CopySheetByName`
- `CopySheet` copies sheet at the specified index
- `CopySheetByName` copies sheet with the specified name
* Make `CopyRelationship` return copied rel and bool flag
* Add func `CopyOverride` to `ContentTypes`
* Add test for `CopyRelationship`
* Add tests for sheet removing funcs
* Add test for `CopyOverride`
* Add tests for `CopySheet`, `CopySheetByName`