Add method Spot.isLabelSet()

When analyzing a Mastodon dataset the ability to know for which spots the user
specified a label can be very helpful.

This makes it easier, for example, to implement Mastodon plugins that
automatically derive a name for spots whose label has not been manually set.

src/main/java/org/mastodon/mamut/model/Spot.java

@@ -167,7 +167,7 @@ public double getBoundingSphereRadiusSquared()
 	@Override
 	public String getLabel()
 	{
-		if ( pool.label.isSet( this ) )
+		if ( isLabelSet() )
 			return pool.label.get( this );
 		else
 			return Integer.toString( getInternalPoolIndex() );
@@ -179,6 +179,11 @@ public void setLabel( final String label )
 		pool.label.set( this, label );
 	}
 
+	public boolean isLabelSet()
+	{
+		return pool.label.isSet( this );
+	}
+
 	@Override
 	public String toString()
 	{