Update Calculation.php

PHPExcel Fatal error: Call to a member function cellExists() line: 3327
Bugfix: Work Item GH-275 - Insert New Row/Column Before is not correctly updating formula references
Bugfix: Work Item GH-275 - Insert New Row/Column Before is not correctly updating formula references
Minor fix to chart layout
Bugfix: Work Item GH-257 - Passing an array of cells to _generateRow() in the HTML/PDF Writer causes caching problems with last cell in the range
Bugfix: Work Item GH-193 - Fix to empty worksheet garbage collection when using cell caching
Bugfix: (Jazzo) Work Item GH-248 - Excel2007 does not correctly mark rows as hidden
Bugfix: (Roy Shahbazian) Work Item GH-299 - Fixed typo in Chart/Layout set/getYMode()
Improve fluent interface in charting methods
Merge pull request #279 from EliuFlorez/patch-1

Update Calculation.php
Bugfix: (EliuFlorez) Work item GH-279 - Fatal error: Call to a member function cellExists() line: 3327 in calculation.php if referenced worksheet doesn't exist
Bugfix: Work Item GH-290 - AdvancedValueBinder "Division by zero"-error
Update to copyright year in file headers
Bugfix: Work Item CP20604 - Adding Sheet to Workbook Bug