update analysis scripts and input files for xrb_spherical problem

[zhichen3]

PR summary

1. makes the slice script more robust.
2. add a flame tracking script (tracking_front.py) that finds the flame front in terms of theta
3. add a script that produce all the slice plots using output of the tracking_front script. Mainly for making a movie.
4. update some simulation input files.

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