Allow compressed Gifti images

Allow .gz and .bz2 compressed Gifti images.

Closes gh-1071

Author: matthew-brett

nibabel/gifti/gifti.py

@@ -565,6 +565,7 @@ class GiftiImage(xml.XmlSerializable, SerializableImage):
     """
     valid_exts = ('.gii',)
     files_types = (('image', '.gii'),)
+    _compressed_suffixes = ('.gz', '.bz2')
 
     # The parser will in due course be a GiftiImageParser, but we can't set
     # that now, because it would result in a circular import.  We set it after

nibabel/gifti/tests/data/external.gii.bz2

@@ -462,3 +462,11 @@ def test_external_file_failure_cases():
     parser = GiftiImageParser()
     with pytest.raises(GiftiParseError):
         img = parser.parse(xmldata)
+
+
+def test_load_compressed():
+    for ext in ('', '.gz', '.bz2'):
+        fn = pjoin(IO_DATA_PATH, 'external.gii' + ext)
+        img7 = load(fn)
+        assert_array_almost_equal(img7.darrays[0].data, DATA_FILE7_darr1)
+        assert_array_almost_equal(img7.darrays[1].data, DATA_FILE7_darr2)