PrismLog: implement Closable / AutoClosable [with Steffen Maercker]

This allows for PrismLogs to be used in
try (PrismLog log = ...) {
  ...
}
blocks (Java 7 style resource blocks). When leaving the try block,
the log is automatically closed. This can be helpful in long-running
processes to get a look at a PrismFileLog output, as closing the log
forces a flush as well. 


git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@10574 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/prism/PrismLog.java

@@ -27,7 +27,9 @@
 
 package prism;
 
-public abstract class PrismLog
+import java.io.Closeable;
+
+public abstract class PrismLog implements Closeable, AutoCloseable
 {
 	/**
 	 * Specifies that only more important messages should be printed