adding overrides to FERC-EIA appends instead of replaces

[cmgosnell]

when looking into duke's data, I realized that our training data overrides for the FERC-EIA process is effectively appending new records as opposed to replacing them.

Here's a little snippet of the output:

this happens in pudl_rmi.connect_ferc1_to_eia.override_best_match_with_training_df()