address rotation source in spherical 2d coordinate

[zhichen3]

PR summary

This addresses #2927 regarding the rotation bit.
I mainly separated get_omega() to return magnitude of the angular frequency,
then made get_omega_vec() to depend on j, since $\Omega \hat{z} = \Omega[\cos(\theta) \hat{r} - \sin(\theta) \hat{\theta})]$.
To deal with the problem of position vector, I just made sure r is (r,0,0) before the cross_product call.
I also made computing dt_omega_matrix into a separate function since omega vector now varies.

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