bennu 0.3.0

• kit_summary_table created to provide a quick way of summarizing model
  output by different strata

• model_random_walk_data created to more closely mimic Bayesian data
  generating process

• generate_model_data deprecated as model_random_walk_data will supplant it
  as way of generating simulation data to test properties of the model

• Updates to stan model to make it compliant to rstan 2.26 (#22)