pyDKB/*/consumer: make FileConsumer._filenames_form_dir() generator.

Not a big deal, but most of FileConsumer methods, related to input
files, are generators -- so why not. Now instead of checking all files
in the directory and creating a list of those with proper extension, it
yields first file name that fits requirements, and checks next file only
when asked about it.

Author: mgolosova

Utils/Dataflow/pyDKB/dataflow/communication/consumer/FileConsumer.py

@@ -111,9 +111,9 @@ def _filenames_from_dir(self, dirname):
                 if os.path.isfile(os.path.join(dirname, f)) \
                         and f.lower().endswith(ext):
                     files.append(f)
+                    yield f
         except OSError, err:
             raise Consumer.ConsumerException(err)
-        return files
 
     def _adjusted_filenames(self):
         """ Return iterable object, yielding filename and path to file. """