Renamed some of the internal routines for consistency.

Renamed the argument to the `update` routines from `name` to `path` (it is really a path).
The `update` routines now use the new add_by_path feature so if the variable is not present, it will be added using the path (formerly, it added it as a name, which was inconsistent). This is a minor break of backward compatibility.
Add unicode version of add_scalar_by_path.

Author: jacobwilliams