returns the matrix with all entries below a certain threshold
        (i.e. tol) set to zero

def zeroed(self, tol=1e-3):
        """
        returns the matrix with all entries below a certain threshold
        (i.e. tol) set to zero
        """
        new_tensor = self.copy()
        new_tensor[abs(new_tensor) < tol] = 0
        return new_tensor