Add boilerplate and initial activity classes.

src/pvv/side/activity.php

@@ -0,0 +1,14 @@
+<?php declare(strict_types=1);
+namespace pvv\side;
+
+use \DateTime;
+
+interface Activity {
+
+	public function getName(); /* : string */
+
+	public function getLocation(); /* : Location */
+
+	public function getOrganiser(); /* : User */
+
+}