MDL-56046 core_dataformat: added functions to support multiple sheets

Also removed write_header() and write_footer(). The reason for this
is because in core we want to know if the format being used supports
multiple sheets. To do this, and ensure we do not break third-party
dataformat plugins, we are using method_exist(). If write_header()
and write_footer() remain in the base class then this will always
be true.

lib/classes/dataformat/base.php

@@ -76,8 +76,6 @@ abstract class base {
      * Output file headers to initialise the download of the file.
      */
     public function send_http_headers() {
-        global $CFG;
-
         if (defined('BEHAT_SITE_RUNNING')) {
             // For text based formats - we cannot test the output with behat if we force a file download.
             return;
@@ -98,11 +96,18 @@ abstract class base {
     }
 
     /**
-     * Write the start of the format
+     * Write the start of the file.
+     */
+    public function start_output() {
+        // Override me if needed.
+    }
+
+    /**
+     * Write the start of the sheet we will be adding data to.
      *
      * @param array $columns
      */
-    public function write_header($columns) {
+    public function start_sheet($columns) {
         // Override me if needed.
     }
 
@@ -115,12 +120,18 @@ abstract class base {
     abstract public function write_record($record, $rownum);
 
     /**
-     * Write the end of the format
+     * Write the end of the sheet containing the data.
      *
      * @param array $columns
      */
-    public function write_footer($columns) {
+    public function close_sheet($columns) {
         // Override me if needed.
     }
 
+    /**
+     * Write the end of the file.
+     */
+    public function close_output() {
+        // Override me if needed.
+    }
 }