Make ModelSerializer public

This change allows other classes to write & read spots into "raw" format.
Being able to access this class is useful when implementing code
that serializes a Mastodon project into as custom file format. This change
is helpful for implementing a optimized file format in mastodon-git.

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

@@ -33,7 +33,7 @@
 import org.mastodon.graph.ref.AbstractVertexPool;
 import org.mastodon.pool.PoolObjectAttributeSerializer;
 
-class ModelSerializer implements GraphSerializer< Spot, Link >
+public class ModelSerializer implements GraphSerializer< Spot, Link >
 {
 	private ModelSerializer()
 	{}