add Suhonen complexity score overfitting test

[Suhonen(2017)](http://www.iijournals.com/doi/abs/10.3905/jpm.2017.43.2.090?journalCode=jpm) creates a complexity score metric for evaluating backtest overfitting.

We should integrate this into quantstrat.

No files, tests, or entry points are named. Start by reading the linked Suhonen (2017) paper and locating quantstrat's existing backtest performance metrics. Define how the complexity score should fit the package and what test demonstrates that the metric is calculated correctly.

Written by the indexing model from the issue text.