Feature/stat ellipse

[JoFrhwld]

Basically #878, but merged commits from master.

[hadley]

Can you please back out this change?

[hadley]

• Motivate the change in one paragraph, and include it in NEWS.
  In parentheses, reference your github user name and this issue:
  (@hadley, #1234)
• Check pull request only includes relevant changes.
• Use the official style.
• Update documentation and re-run roxygen2
• Add minimal example, if new graphical feature

[JoFrhwld]

I think I've hit the major points.

• Added a brief paragraph to NEWS
• Cleaned up the style
• Rolled back the changes to .Rbuildignore
• Refactored code so that the ellipse calculation is done by a separate function, calculate_ellipse().
• Included reference to the car package.

[hadley]

Looks good - thanks!

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