update scripts and input files for diffusion test

[zhichen3]

PR summary

1. corrects README
2. Update analysis script
3. add spherical geometry input file

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]

spherical input file assumes 1 level (max_level=0) for now.

[zingale]

maybe we should rename cyl in the inputs file to axisymmetric, so it is clear.

Note that in Sedov, we also use cyl to mean a cylindrical explosion in 2D Cartesian and sph to mean a spherical explosion in 2D axisymmetric.

[zhichen3]

done