Performance Analyzer fixes and features

[stereohelix]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Fix: Sharpe ratio calculated according to the standard meaning.

Features:

• Added total exposure as a percentage to backtest report
• Added expected downside (as Bessel-corrected downside quartile) to backtest report

• What is the current behavior? (You can also link to an open issue here)

Incorrect Sharpe: #2064

• What is the new behavior (if this is a feature change)?

More better backtest stats! :)

• Other information:

[askmike]

👍