Added Workflow.get_data_file_name() which returns the full path to a given

data file, or None if the file does not exist. This is tested in the test
workflow.

fluents/workflow.py

@@ -63,6 +63,19 @@ class Workflow:
         self.stages_by_id = {}
         self.app = app
 
+    def get_data_file_name(self, filename):
+        """Checks if a file with the given name exists in the data directory.
+        Returns the file name if the file exists in the data directory, which
+        is defined as datadir/workflowname. If the file does not exist, or the
+        workflow does not have an identificator, this method returns None."""
+        print os.path.join(self.app.options.datadir, self.ident, filename)
+        if self.ident == None:
+            return None
+        fn = os.path.join(self.app.options.datadir, self.ident, filename)
+        if os.path.isfile(fn):
+            return fn
+        return None
+
     def add_stage(self, stage):
         self.stages.append(stage)
         self.stages_by_id[stage.id] = stage