* Implementation of the Excel SUMIFS function
* Fixes for Readers to handle various malformed spreadsheet file formats
* Better error handling in Iterators
* Suppression of Chart formula evaluation on save if formula evaluation is disabled
* Changes for PHP7
* Hopefully fixed a memory issue with unsetting PHPExcel object failing to unset the calculation engine
XMLWriter is surprisingly a different extension from xml; and so is not installed on some systems (such as Gentoo). This commit enforces it to be installed, since it's necessary for the functionality of PHPExcel.