Use UTF-8 encoding in core_metrics

[rolandramp]

I had some problems loading the fira-22.baseline-qrels.tsv, so I did a: "with open(path, 'r', encoding="utf-8")" in the load_qrels function to avoid encoding problems. But maybe this is only a problem, because I am working on a Windows PC.

[ldosen00]

Feel free to state which specific problems you had, but specifically stating the encoding parameter is quite normal.