Allow for adding reference plots to a comparison graph

[chrispcampbell]

When reviewing comparison graphs in a model-check report, it is sometimes helpful to see additional datasets directly inside the comparison graph for reference purposes.

I will add a new optional method to the ComparisonDatasetOptions interface that allows for this:

/**
 * Describes an extra plot to be shown in a comparison graph.
 */
export interface ComparisonPlot {
  /** The dataset key for the plot. */
  datasetKey: DatasetKey
  /** The plot color. */
  color: string
  /** The plot style.  If undefined, defaults to 'normal'. */
  style?: 'normal' | 'dashed'
  /** The plot line width, in px units.  If undefined, a default width will be used. */
  lineWidth?: number
}

export interface ComparisonDatasetOptions {
  // ...

  /**
   * An optional function that allows for including additional reference plots
   * on a comparison graph for a given dataset and scenario.  By default, no
   * additional reference plots are included, but if a custom function is
   * provided, it can return an array of `ComparisonPlot` objects.
   */
  referencePlotsForDataset?: (dataset: ComparisonDataset, scenario: ComparisonScenario) => ComparisonPlot[]
}

Example from the sample-check-app: