Do not cache next interpolation as member

This ensures that next works even when new interpolations are added.

Also test it

pom.xml

@@ -193,5 +193,11 @@
 			<artifactId>scijava-listeners</artifactId>
 			<version>1.0.0-beta-2</version>
 		</dependency>
+		<!-- test dependencies -->
+		<dependency>
+			<groupId>junit</groupId>
+			<artifactId>junit</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 </project>

src/main/java/bdv/viewer/Interpolation.java

@@ -37,15 +37,8 @@ public enum Interpolation
 	NEARESTNEIGHBOR( "nearest-neighbor interpolation" ),
 	NLINEAR( "tri-linear interpolation" );
 
-	static {
-		NEARESTNEIGHBOR.next = NLINEAR;
-		NLINEAR.next = NEARESTNEIGHBOR;
-	}
-
 	private final String name;
 
-	private Interpolation next = null;
-
 	private Interpolation( final String name )
 	{
 		this.name = name;
@@ -57,6 +50,6 @@ public enum Interpolation
 	}
 
 	public Interpolation next() {
-		return next;
+		return Interpolation.values()[ ( this.ordinal() + 1 ) % Interpolation.values().length ];
 	}
 }

src/test/java/bdv/viewer/InterpolationTest.java

+package bdv.viewer;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class InterpolationTest {
+
+    @Test
+    public void next() {
+        Assert.assertEquals( Interpolation.NLINEAR, Interpolation.NEARESTNEIGHBOR.next() );
+        Assert.assertEquals( Interpolation.NEARESTNEIGHBOR, Interpolation.NLINEAR.next() );
+    }
+}