Modify postprocessing/comstock.py and cbecs.py to use polars instead of pandas except for plotting

[asparke2]

Currently, we use a mix of Pandas and Polars in comstock-postprocessing. Change this all over to Polars. It performs better, and having one library to manage changes in would be nice. The exception to this is converting to Pandas right before plotting, because Seaborn works natively with Pandas.

[mdahlhausen]

I suggest we wait until polars has a version 1 before doing this; polars is much more unstable, and dealing with break changes may be slower than the minor additional run time in existing code.