add check for proper class hiearchy in table syntax

Plugins extend this class, but may not inherit from Aggregation, yet.
A proper error should be shown.

Author: splitbrain

syntax/table.php

@@ -115,8 +115,12 @@ public function render($format, Doku_Renderer $renderer, $config)
                 $search->setOffset(0);
             }
 
-            /** @var Aggregation $table */
             $table = new $this->tableclass($mainId, $format, $renderer, $search);
+            if (!is_a($table, Aggregation::class)) {
+                // this may happen with plugins that extend struct
+                throw new StructException('Aggregation class does not inherit Aggregation: ' . $this->tableclass);
+            }
+
             $table->startScope();
             $table->render(true);
             $table->finishScope();