address timestep and cfl violation for Spherical2D

[zhichen3]

PR summary

address #2927 . Change timestep and cfl_violation so we use rdtheta, rdx[1], which is the physical cell length instead of just dx[1] for cell length in theta direction for spherical 2d.

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

[zhichen3]

we might need to change things in radiation, especially since dx.array is passed on to estimate_gamrPr. I could just change it to rdtheta, but I don't really know if its the right thing to do. It's probably only supported for cartesian?