1. implemented XSSFSheet.autosizeColumn(), for now mostly duplicated HSSF code, will...

1. implemented XSSFSheet.autosizeColumn(), for now mostly duplicated HSSF code, will be refactored in future.2. fixed bug #45974: XSSFCell.getCellStyle can return null3. more code cleanup and reaftoring, removed usages of obsolete XSSFCell.getCellNum() in favor of XSSFCell.getColumnIndex(), also more javadoc in core classes
4. fixed a blocker: calling XSSFSheet.getNumMergedRegions() resulted in unreadable workbook, this methods structurally modified worksheet and added unnecessary data

git-svn-id: https://svn.apache.org/repos/asf/poi/branches/ooxml@706691 13f79535-47bb-0310-9956-ffa450edef68