[YAML] Append rather than replace extra description

interfaces/cython/cantera/ck2yaml.py

@@ -39,8 +39,8 @@
 is used to override default phase names (i.e. 'gas').
 
 The '--extra=<filename>' option takes a YAML file as input. This option can be
-used to specify an alternative file description, or to define custom fields that
-are included in the YAML output.
+used to add to the file description, or to define custom fields that are 
+included in the YAML output.
 """
 
 from collections import defaultdict, OrderedDict
@@ -1355,7 +1355,9 @@ def load_extra_file(self, path):
         # replace header lines
         if 'description' in yml:
             if isinstance(yml['description'], str):
-                self.header_lines = yml.pop('description').split('\n')
+                if self.header_lines:
+                    self.header_lines += ['']
+                self.header_lines += yml.pop('description').split('\n')
             else:
                 raise InputError("The alternate description provided in "
                     "'{}' needs to be a string".format(path))

interfaces/cython/cantera/test/test_convert.py

@@ -454,9 +454,8 @@ def test_extra(self):
         with open(output, 'rt', encoding="utf-8") as stream:
             yml = yaml.safe_load(stream)
 
-        desc = yml['description']
-        self.assertEqual(yml['description'],
-                         'This is an alternative description.')
+        desc = yml['description'].split()[-1]
+        self.assertEqual(desc, 'This is an alternative description.')
         for key in ['foo', 'bar']:
             self.assertIn(key, yml.keys())