Continuing the work from #2828, #2841, #2846, and #2852. This is probably my last change in this area for a while.
Bubble charts can have bubbles of different sizes. Phpspreadsheet had not supported this. Openpyxl comes with sample code to generate such a chart. I was especially drawn to that solution because its namespace usage would have been unexpected before 2852. And it turned out to come with other surprises - use of absolute paths in the .rels files (PhpSpreadsheet expected only relative), use of a one-cell anchor to place the chart (PhpSpreadsheet expected two-cell anchor or absolute positioning), plaintext in the legend (Phpspreadsheet expected RichText), no cached values for chart data. Excel handles the file okay, and this PR makes sure PhpSpreadsheet does as well. This file is now Samples/Templates/32readwriteBubbleChart2, and is used for both generating a sample output file and in formal tests. A new sample in the 33* series demonstrates how to code these.
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