Reorganize Samples

The samples have become unwieldy when running them from a browser. In particular, the drop-down lists are fixed size with no scrolling, and many of them are now just too large. I have moved all the Calculation samples up a level, and broken several categories (Basic, Chart, DateTime, Engineering, Financial, and Reader) into several pieces.

Convert-Online (now found in the Engineering category) had a number of different problems which are now resolved. It is the only member with any significant code change.

samples/DateTime2/TODAY.php

@@ -0,0 +1,29 @@
+<?php
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+
+require __DIR__ . '/../../Header.php';
+
+$category = 'Date/Time';
+$functionName = 'TODAY';
+$description = 'Returns the serial number of the current date';
+
+$helper->titles($category, $functionName, $description);
+
+// Create new PhpSpreadsheet object
+$spreadsheet = new Spreadsheet();
+$worksheet = $spreadsheet->getActiveSheet();
+
+$worksheet->setCellValue('A1', '=TODAY()');
+$worksheet->getStyle('A1')
+    ->getNumberFormat()
+    ->setFormatCode('yyyy-mm-dd');
+
+// Test the formulae
+$helper->log(
+    'Today is '
+    . $worksheet->getCell('A1')->getCalculatedValue()
+    . ' ('
+    . $worksheet->getCell('A1')->getFormattedValue()
+    . ')'
+);