feat: make IoPolicy public for use in other KVA implementations

Signed-off-by: Caleb Hulbert <cmhulbert@gmail.com>

Author: cmhulbert

src/main/java/org/janelia/saalfeldlab/n5/IoPolicy.java

@@ -5,7 +5,7 @@
 
 import java.io.IOException;
 
-interface IoPolicy {
+public interface IoPolicy {
 
     void write(String key, ReadData readData) throws IOException;