Add boilerplate and initial activity classes.

src/pvv/side/repeatingactivity.php

@@ -0,0 +1,12 @@
+<?php declare(strict_types=1);
+namespace pvv\side;
+
+use \DateTime;
+
+interface RepeatingActivity extends Activity {
+
+	public function getNextEventFrom(DateTime $date) /* : Event */;
+
+	public function getPreviousEventFrom(DateTime $date) /* : Event */;
+
+}