Made period register variable final

contrib/src/main/java/gov/nasa/jpl/aerie/contrib/models/SampledResource.java

@@ -16,7 +16,7 @@
 public class SampledResource<T> implements DiscreteResource<T> {
   private final Register<T> result;
   private final Supplier<T> sampler;
-  private Register<Double> period = Register.forImmutable(1.0);
+  private final Register<Double> period = Register.forImmutable(1.0);
 
   /**
    * Constructor that does not require caller to specify a period and therefore assumes a sample