`polars` for data file conversion too

Before #4, `pandas` was our go-to choice for handling the Excel file-formatted data that the connectivity information was given to us in. #2 currently exists to turn this into a proper workflow.

Once we have #2 working and merged (using `pandas`), we should then convert the functions across to use `polars` instead - which we suspect will be faster and also avoids us using two different dataframe libraries.

Start by reading issue #2 and the existing pandas-based workflow referenced there, then review the functions that handle the Excel-formatted connectivity data. This work should wait until #2 is merged; it is done when those functions have been converted to polars without retaining two dataframe libraries.

Written by the indexing model from the issue text.