added channel constant to pnml constants

src/data/java/de/tudresden/inf/st/pnml/base/constants/PnmlConstants.java

@@ -17,6 +17,7 @@ public final class PnmlConstants {
 
     // general properties
     public static final String NODE_KEY = "node";
+    public static final String CHANNEL = "channel";
     public static final String TYPE_KEY = "type";
     public static final String TOOL_SPEC_KEY = "de.tudresden.inf.st.pnml.distributedPN";