Fixes #21. Read-only access now works.

A regenerated Treant tries to apply any necessary schema updates to its
state file upon init, but when loading read-only Treants this will yield
an exception. Thus, if updating fails, the Treant now lets it slide and
will work as normal (read-only methods will work).

datreant/persistence.py

@@ -756,8 +756,13 @@ def __init__(self, filename, logger=None, **kwargs):
         """
         super(TreantFile, self).__init__(filename, logger=logger)
 
-        # if file does not exist, it is created
-        self.create(**kwargs)
+        # if file does not exist, it is created; if it does exist, it is updated
+        try:
+            self.create(**kwargs)
+        except OSError:
+            # in case the file is read-only; we can't update but may still want
+            # to use it
+            pass
 
     def _open_file_r(self):
         return tables.open_file(self.filename, 'r')