v0.2.8

• Improve Stan models for shorter run times
• Add dgmrp() for fitting single-issue MRP models with hierarchical covariates
• Add class dgmrp_fit for models fitted with dgmrp(), inheriting from a new
  virtual class dgo_fit
• dgirt() now returns a dgirt_fit-class object that also inherits from
  dgo_fit class
• Bugfixes