Expose a Sheet object on Reader::XLSX::nextSheet()

Added Sheet class for the XLSX reader that exposes basic sheet info, such as name or ID.
When retrieving the sheet data XML, added extra XML parsing to retrieve sheet data.
Added test

src/Spout/Reader/XLSX.php

@@ -76,7 +76,7 @@ class XLSX extends AbstractReader
             $this->extractSharedStrings($filePath);
 
             // Fetch all available worksheets
-            $this->worksheetHelper = new WorksheetHelper($filePath);
+            $this->worksheetHelper = new WorksheetHelper($filePath, $this->globalFunctionsHelper);
             $this->worksheets = $this->worksheetHelper->getWorksheets($filePath);
 
             if (count($this->worksheets) === 0) {
@@ -119,29 +119,31 @@ class XLSX extends AbstractReader
      * Moves the pointer to the current worksheet.
      * Moving to another worksheet will stop the reading in the current worksheet.
      *
-     * @return void
+     * @return \Box\Spout\Reader\Sheet The next sheet
      * @throws Exception\ReaderNotOpenedException If the stream was not opened first
      * @throws Exception\EndOfWorksheetsReachedException If there is no more worksheets to read
      */
     public function nextSheet()
     {
-        if ($this->hasNextSheet()) {
-            if ($this->currentWorksheet === null) {
-                $nextWorksheet = $this->worksheets[0];
-            } else {
-                $currentWorksheetNumber = $this->currentWorksheet->getWorksheetNumber();
-                $nextWorksheet = $this->worksheets[$currentWorksheetNumber + 1];
-            }
-
-            $this->initXmlReaderForWorksheetData($nextWorksheet);
-            $this->currentWorksheet = $nextWorksheet;
-
-            // make sure that we are ready to read more rows
-            $this->hasReachedEndOfFile = false;
-            $this->emptyRowDataBuffer();
-        } else {
+        if (!$this->hasNextSheet()) {
             throw new EndOfWorksheetsReachedException('End of worksheets was reached. Cannot read more worksheets.');
         }
+
+        if ($this->currentWorksheet === null) {
+            $nextWorksheet = $this->worksheets[0];
+        } else {
+            $currentWorksheetNumber = $this->currentWorksheet->getWorksheetNumber();
+            $nextWorksheet = $this->worksheets[$currentWorksheetNumber + 1];
+        }
+
+        $this->initXmlReaderForWorksheetData($nextWorksheet);
+        $this->currentWorksheet = $nextWorksheet;
+
+        // make sure that we are ready to read more rows
+        $this->hasReachedEndOfFile = false;
+        $this->emptyRowDataBuffer();
+
+        return $this->currentWorksheet->getExternalSheet();
     }
 
     /**