Add missing getID method

Context.getId() method missing.

src/ToolProvider/Context.php

@@ -10,7 +10,7 @@ use IMSGlobal\LTI\ToolProvider\Service;
  * @author  Stephen P Vickers <svickers@imsglobal.org>
  * @copyright  IMS Global Learning Consortium Inc
  * @date  2016
- * @version 3.0.0
+ * @version 3.0.1
  * @license http://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
  */
 class Context
@@ -182,6 +182,18 @@ class Context
 
     }
 
+/**
+ * Get context ID.
+ *
+ * @return string ID for this context.
+ */
+    public function getId()
+    {
+
+        return $this->ltiContextId;
+
+    }
+
 /**
  * Get the context record ID.
  *