ODS writer

Added ODS writer
Refactored XLSX writer to abstract some pieces into an abstract multi-sheets writer
Created an abstract style helper
Moved shared components around

src/Spout/Writer/Common/Internal/WorksheetInterface.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Box\Spout\Writer\Common\Internal;
+
+/**
+ * Interface WorksheetInterface
+ *
+ * @package Box\Spout\Writer\Common\Internal
+ */
+interface WorksheetInterface
+{
+    /**
+     * @return \Box\Spout\Writer\Common\Sheet The "external" sheet
+     */
+    public function getExternalSheet();
+
+    /**
+     * @return int The index of the last written row
+     */
+    public function getLastWrittenRowIndex();
+
+    /**
+     * Adds data to the worksheet.
+     *
+     * @param array $dataRow Array containing data to be written.
+     *          Example $dataRow = ['data1', 1234, null, '', 'data5'];
+     * @param \Box\Spout\Writer\Style\Style $style Style to be applied to the row. NULL means use default style.
+     * @return void
+     * @throws \Box\Spout\Common\Exception\IOException If the data cannot be written
+     * @throws \Box\Spout\Common\Exception\InvalidArgumentException If a cell value's type is not supported
+     */
+    public function addRow($dataRow, $style);
+
+    /**
+     * Closes the worksheet
+     *
+     * @return void
+     */
+    public function close();
+}