describe_zernike(11) is wrong

[jmeyers314]

Prints

sqrt(5) * ( - 6y^2 + 6y^4 - 6x^2 + 12x^2y^2 + 6x^4)

instead of

sqrt(5) * (1 - 6y^2 + 6y^4 - 6x^2 + 12x^2y^2 + 6x^4).

[jmeyers314]

See #1358 .