Add slippage and R^2

[keo]

• Getting first strategies working
• First 2 strategies completed backtest - metrics need work
• We now have backtests and tearsheets. Metrics still sus
• We now have backtests and tearsheets. Metrics good now, hopefully
• Add markdown descriptions
• Vectorise our backtesting 🚀
• Almost have a contribution framework working
• add slippage
• add r^2 to results and linear regression to equity curve