PolarBasis2D only works for `n_theta` even

[j-c-c]

PolarBasis2D requires n_theta to be even for its evaluate() method. We need to check if this can be refactored to work for odd values of n_theta.

[janden]

I don't know if we have an obvious use case for this, so I would move this down the priority list quite a bit.

[garrettwrong]

Fair enough. Thanks. I'll pop it off 10.2 project then.