Clean unnecessary attributes from the read

this gives on average 40% file size reduction.

public/java/src/org/broadinstitute/sting/utils/sam/GATKSAMRecord.java

@@ -241,4 +241,10 @@ public class GATKSAMRecord extends BAMRecord {
     public boolean isEmpty() {
         return this.getReadLength() == 0;
     }
+
+    public void simplify () {
+        GATKSAMReadGroupRecord rg = getReadGroup();
+        this.clearAttributes();
+        setReadGroup(rg);
+    }
 }