Update block.py

Added a call to `text_file.close()` after the `np.savetxt()` in `WriteAsciiBlock`.

python/bifrost/block.py

@@ -571,6 +571,7 @@ def main(self, input_ring):
                 data_accumulate = unpacked_data[0]
         text_file = open(self.filename, 'a')
         np.savetxt(text_file, data_accumulate.reshape((1, -1)))
+        text_file.close()
 class CopyBlock(TransformBlock):
     """Copies input ring's data to the output ring"""
     def __init__(self, gulp_size=1048576):