Added successive differences contrast coding and Cook's distance diagnostic plot

[acp29]

Added:

• Successive differences contrast coding (sdif)
• Cook's distance diagnostic plot
• Horizontal lines/thresholds to the "Spread-Location" plot to indicate where absolute standardized residuals equal 2, 3 or 4. Useful to identify potential outliers.
• Horizontal lines/thresholds to the "Influential values" plot to indicate where Cook's distance exceeds 4/dfe, 0.5 or 1.