Add basic versioning to bulkdata table format file.

nilmdb/bulkdata.py

@@ -184,7 +184,8 @@ class Table(object):
 
         format = { "rows_per_file": rows_per_file,
                    "files_per_dir": files_per_dir,
-                   "struct_fmt": struct_fmt }
+                   "struct_fmt": struct_fmt,
+                   "version": 1 }
         with open(os.path.join(root, "_format"), "wb") as f:
             pickle.dump(format, f, 2)
 
@@ -196,6 +197,11 @@ class Table(object):
         # Load the format and build packer
         with open(os.path.join(self.root, "_format"), "rb") as f:
             format = pickle.load(f)
+
+        if format["version"] != 1: # pragma: no cover (just future proofing)
+            raise NotImplementedError("version " + format["version"] +
+                                      " bulk data store not supported")
+
         self.rows_per_file = format["rows_per_file"]
         self.files_per_dir = format["files_per_dir"]
         self.packer = struct.Struct(format["struct_fmt"])