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:

• [x] - fit_seromodel() (#213)
• [x] - build_stan_data() (#232)
  • [x] set_stan_data_defaults()
  • [x] get_foi_index() (#206)
• [x] plot_seromodel() (#215)
  • [x] plot_summary()
  • [x] plot_seroprevalence_estimates()
  • [x] plot_serosurvey()
  • [x] plot_foi_estimates()
  • [x] plot_rhats()(#226 )