Implement backfill cohorts simulations

[danielinteractive]

• Adapt Data class
  • backfilled and response slots
• Adapt update method for Data
  • adding response, backfill and cohort arguments
• Amend plot method for Data
  • how to best show response and/or backfill status?
• RandomCohortSize class / methods
• Adapt StoppingPatientsNearDose class / method
• Opening classes and openCohort methods:
  • OpeningMinDose
  • OpeningMinCohorts
  • OpeningNone
  • OpeningMinResponses (to use the response efficacy data)
• Recruitment classes and maxRecruits methods:
  • RecruitmentUnlimited
  • RecruitmentRatio
• Backfill class
• tidy methods
  • Data
• Adapt Design class
  • add backfill slot
• Adapt simulate method for Design class
  • add trueResponse argument
  • adapt code - try to move things into helper function to keep function size more or less
• Consider adapting simulate methods for DualDesign and DADesign or assert that there is no backfilling in place for those
• knit_print methods for
  • Opening
  • Recruitment
  • Backfill classes
  • enable again in Design
• Add a new vignette about backfilling with examples and documentation