Unit testing is missing for main functionalities of the package

[ntorresd]

Since we basically rewrote the modelling and plotting functionalities on v.1.0.0, we need to restructure unitary testing. The main functionalities missing unit tests are:

• fit_seromodel() (Add unit test for fit_seromodel() #213)
• build_stan_data()
  • set_stan_data_defaults()
  • get_foi_index() (Adding structure to foi_index in fit_seromodel #206)
• plot_seromodel() (Add unit test for visualisation functionalities #215)
  • plot_summary()
  • plot_seroprevalence_estimates()
    • plot_serosurvey()
  • plot_foi_estimates()
  • plot_rhats()
• prob_infected_* (Stan functions - Unit testing for stan functions #214)