Add tests for densesolver in L2

[mrava87]

Once Pylops v1.18.1 has been released, add the following test to test_norms:

@pytest.mark.parametrize("par", [(par1), (par2)])
def test_L2_dense(par):
    """L2 norm of Op*x with dense Op and proximal/dual proximal
    """
    b = np.zeros(par['nx'], dtype=par['dtype'])
    d = np.random.normal(0., 1., par['nx']).astype(par['dtype'])
    l2 = L2(Op=MatrixMult(np.diag(d), dtype=par['dtype']),
            b=b, sigma=par['sigma'], densesolver='numpy')

    # norm
    x = np.random.normal(0., 1., par['nx']).astype(par['dtype'])
    assert l2(x) == (par['sigma'] / 2.) * np.linalg.norm(d * x) ** 2

    # prox: since Op is a Diagonal operator the denominator becomes
    # 1 + sigma*tau*d[i] for every i
    tau = 2.
    den = 1. + par['sigma'] * tau * d ** 2
    assert_array_almost_equal(l2.prox(x, tau), x / den, decimal=4)