Interactions!

Summary of larger updates:

• Added support for creating interactions among variables
• Removed summary plot function to drop ggplot dependencies

Summary of smaller updates:

• Added a few new examples on how to use the interactions argument.

New fractional factorial experiment design options

• Created new functions for creating different experiment designs:
  • Full factorial
  • "D", "A", and "I" optimal designs
• Modified previous functions for creating a coded survey from a given experiment design
• Changed the output of the sampleSizer function to show the sample size (rather than the number of observations, which can be different since respondents may answer more than one choice question).