Reshaping functionality

[leeper]

Please specify whether your issue is about:

• a possible bug
• a question about package functionality
• a suggested code or documentation change, improvement to the code, or feature request

Add a function to reshape a "wide" conjoint dataset into a "long" conjoint dataset, with some flexibility over how that works.

[leeper]

Mostly there. A few lingering issues:

• test suite
• formula interface for variables in profile_variables and task_variables
• possibly better handling of outcome variables that encode which profile was selected
• add some details to docs about:
  • why this is not a usual reshape-to-long situation (variables vary at three levels: respondent, task, and profile)
  • mention feature order (as something that should be put in task_variables)
  • mention its often easier to recode features after tidying as it reduced duplicated work