As I test new strategies, parsing results.txt by hand gets rather frustrating. For example, I am looking at two of my strategies- nprtt and ultimateDetective- to understand why nprtt is marginally outperforming ultimateDetective, and this is basically impossible with results.txt. With headToHead.csv, I can just import the file into Excel or Google Sheets and see which strategies nprtt does better on than ultimateDetective (and vice versa) so I know what to dive deeper into, in results.txt.

This Pull Request also does a couple other quality of life improvements:

• It autoformats prisonersDilemma.py with Black. I'll be honest, this just happened automatically because of my vim autocommand that I was too lazy to disable. It looks cleaner now and I'm kind of hoping you will not make me undo this.
• It uses decimal.Decimal to represent the average scores within a game (and consequently, overall). Decimal uses fixed-point arithmetic and so doesn't have the imprecision of float. I did this because it makes the headToHead.csv look nicer (floating point imprecision creates some very ugly numbers) without having to give up information by rounding.

You can verify yourself that headToHead.csv is properly parsed by Excel and Google Sheets.