Remove terms with coefficients of absolute value atol or less.

def clean(self: 'TSelf', *, atol: float=1e-9) -> 'TSelf':
        """Remove terms with coefficients of absolute value atol or less."""
        negligible = [v for v, c in self._terms.items() if abs(c) <= atol]
        for v in negligible:
            del self._terms[v]
        return self