Add setName Method for Chart (#3001)

Addresses a problem identified in issue #2991. Chart name is set in constructor, but there is no method to subsequently change it. This PR adds a method to do so.

tests/PhpSpreadsheetTests/Chart/ChartMethodTest.php

@@ -93,8 +93,9 @@ class ChartMethodTest extends TestCase
             $xAxis, // xAxis
             $yAxis // yAxis
         );
-        $chart2 = new Chart('chart1');
+        $chart2 = new Chart('xyz');
         $chart2
+            ->setName('chart1')
             ->setLegend($legend)
             ->setPlotArea($plotArea)
             ->setPlotVisibleOnly(true)