fix divu when theta=0 or pi

[zhichen3]

PR summary

When calculating vy, when $\theta = 0$ or $\theta = \pi$, sinc=0 which causes division by 0. But the numerator should cancel due to azimuthal symmetry. So numerator is 0. So set vy=0 when that happens.

Do it more precisely, expand out divergence in theta to be: $\frac{1}{r sin\theta} \frac{\partial (v sin\theta)}{\partial \theta}$ = $\frac{1}{r} \left(\frac{\Delta v}{\Delta \theta} + v cot\theta \right)$. Both $\Delta v = 0$ and $v = 0$ at $\theta = 0$, so we have $\frac{1}{r} \frac{cos \theta v}{sin \theta} = 0/0$. With l'hopital, we have $lim_{\theta \rightarrow 0} \frac{\partial(v cos \theta) / \partial \theta}{cos \theta } = 0$

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs