Future warning: Pandas functionality change for appending to dataframes.

[marc-vdm]

Pandas will be removing frame.append() as of version 2.0 (we are at 1.4 now). We need to refactor for this sometime in the coming months. Source: https://pandas.pydata.org/pandas-docs/dev/whatsnew/v1.4.0.html#deprecated-frame-append-and-series-append