createLikertData: createLikertData
In GregorDeCillia/shinyLikert: Shiny wrapper for plots of likert items

Create data of class likertData from three dataframes. This function also applies some sanity checks for the inputs.

1 2	createLikertData(likert_scale_questions, participant_factors, question_factors, response_levels)

`likert_scale_questions`	The data consisting of likert-scale items with the response_levels supplied below. The columns should be either factorial or numeric, containing only whole numbers (e.g. 0 - 5, 1 - 5, ... )
`participant_factors`	Factors that can be used to classify the participants
`question_factors`	Factors that can be used to classify the questions
`response_levels`	The answer choices for the likert questions. Often c( "strongly agree", "agree", ... )