Add PivotTable samples

Adds runnable samples under samples/PivotTable demonstrating the pivot table
feature, discovered automatically by the sample browser:

- 01_Create_PivotTable: build a basic pivot (row/column/value fields) from a
  source range and save it as Xlsx.
- 02_PivotTable_Page_Filter: add a page (report filter) field.
- 03_PivotTable_Grouping: group a numeric field into ranges and a date field
  by quarter.
- 04_Read_PivotTable: write a pivot, load it back, and inspect the pivot table
  object model (source, row/column/page fields, value aggregation).

Directory is named PivotTable to match the PascalCase convention used by the
other sample categories (Table, Chart, ...).

samples/PivotTable/01_Create_PivotTable.php

@@ -0,0 +1,62 @@
+<?php
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PhpOffice\PhpSpreadsheet\Worksheet\PivotTable\PivotField;
+use PhpOffice\PhpSpreadsheet\Worksheet\PivotTable\PivotTableBuilder;
+
+require __DIR__ . '/../Header.php';
+/** @var PhpOffice\PhpSpreadsheet\Helper\Sample $helper */
+
+// Create new Spreadsheet object
+$helper->log('Create new Spreadsheet object');
+$spreadsheet = new Spreadsheet();
+$spreadsheet->getProperties()
+    ->setCreator('PhpSpreadsheet')
+    ->setTitle('PhpSpreadsheet PivotTable Sample')
+    ->setCategory('PivotTable');
+
+// Fill a worksheet with the source data for the pivot table.
+$helper->log('Add source data');
+$dataSheet = $spreadsheet->getActiveSheet();
+$dataSheet->setTitle('Data');
+$dataSheet->fromArray(
+    [
+        ['Region', 'Product', 'Quarter', 'Amount'],
+        ['East', 'Widget', 'Q1', 100],
+        ['East', 'Gadget', 'Q1', 200],
+        ['West', 'Widget', 'Q1', 150],
+        ['West', 'Gadget', 'Q1', 250],
+        ['East', 'Widget', 'Q2', 120],
+        ['East', 'Gadget', 'Q2', 220],
+        ['West', 'Widget', 'Q2', 170],
+        ['West', 'Gadget', 'Q2', 270],
+    ],
+    null,
+    'A1'
+);
+
+// Create a worksheet to hold the pivot table.
+$pivotSheet = $spreadsheet->createSheet();
+$pivotSheet->setTitle('PivotTable');
+
+// Build a pivot table:
+//   rows    = Region
+//   columns = Product
+//   values  = Sum of Amount
+$helper->log('Build pivot table');
+$builder = new PivotTableBuilder($dataSheet, 'A1:D9');
+$builder
+    ->addRowField('Region')
+    ->addColumnField('Product')
+    ->addDataField('Amount', PivotField::SUBTOTAL_SUM);
+
+$pivotTable = $builder->build($pivotSheet, 'A3', 'SalesByRegion');
+
+$helper->log('Pivot table "' . $pivotTable->getName() . '" created at ' . $pivotTable->getLocation());
+$helper->log(
+    'The pivot table is written with refresh-on-load set, so the spreadsheet '
+    . 'application fills in the summarised values when the file is opened.'
+);
+
+// Pivot tables are an Xlsx feature.
+$helper->write($spreadsheet, __FILE__, ['Xlsx']);