Extend LeastSquaresMovingAverage to Accept a Benchmark

Expected Behavior

Lean supports LeastSquaresMovingAverage as a Lean Indicator which takes a Symbol object, a benchmark Symbol, and a look-back period.

Actual Behavior

The current implementation assumes a constant slope: LeastSquaresMovingAverage.cs#L59. See #83.

Potential Solution

Implement this indicator.

Checklist

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