Return a list of values associated to a particular list of keys.

def get_values(self, keys):
        """
        Return a list of values associated to a particular list of keys.
        """
        if is_string(keys):
            return [s.__dict__[keys] for s in self.sections]
        else:
            values = []
            for k in keys:
                values.append([s.__dict__[k] for s in self.sections])
            return values