Add hierarchical to the class javadoc of the classification class

src/main/java/org/mastodon/mamut/classification/util/Classification.java

@@ -41,7 +41,7 @@
  * A class that encapsulates the result of a hierarchical clustering algorithm.<br>
  * It contains:
  *     <ul>
- *         <li>the root {@link Cluster} object, from which the results of the algorithm can be accessed</li>
+ *         <li>the root {@link Cluster} object, from which the results of the algorithm can be accessed. Clusters are hierarchically organized.</li>
  *         <li>a {@link List} of {@link ObjectClassification} objects, where each objects contains:
  *         	  <ul>
  *               <li>a {@link Cluster} object, which represents the classified objects in the dendrogram</li>