Add convenient method for checking if object is MatrixProject

This is useful for jelly pages where instanceof operator doesn't work.

core/src/main/java/hudson/Functions.java

@@ -28,6 +28,7 @@ package hudson;
 import hudson.cli.CLICommand;
 import hudson.console.ConsoleAnnotationDescriptor;
 import hudson.console.ConsoleAnnotatorFactory;
+import hudson.matrix.MatrixProject;
 import hudson.model.AbstractProject;
 import hudson.model.Action;
 import hudson.model.Describable;
@@ -187,6 +188,10 @@ public class Functions {
     public static boolean isModelWithChildren(Object o) {
         return o instanceof ModelObjectWithChildren;
     }
+    
+    public static boolean isMatrixProject(Object o) {
+        return o instanceof MatrixProject;
+    }
 
     public static String xsDate(Calendar cal) {
         return Util.XS_DATETIME_FORMATTER.format(cal.getTime());