Read a list of variables/dimensions from file. If a key is not present the corresponding
        entry in the output dictionary is set to None.

def read_keys(self, keys, dict_cls=AttrDict, path="/"):
        """
        Read a list of variables/dimensions from file. If a key is not present the corresponding
        entry in the output dictionary is set to None.
        """
        od = dict_cls()
        for k in keys:
            try:
                # Try to read a variable.
                od[k] = self.read_value(k, path=path)
            except self.Error:
                try:
                    # Try to read a dimension.
                    od[k] = self.read_dimvalue(k, path=path)
                except self.Error:
                    od[k] = None

        return od