feat: add access to underlying measure for sub-classes

Author: metacosm

measure/src/main/java/net/laprun/sustainability/power/measure/StoppedPowerMeasure.java

@@ -63,4 +63,8 @@ public TimestampedMeasures getNthTimestampedMeasures(int n) {
     private int ensureIndex(int upToIndex) {
         return Math.min(upToIndex, samples - 1);
     }
+
+    protected OngoingPowerMeasure underlyingMeasure() {
+        return measure;
+    }
 }