Add options to plot single FOI estimates and rhats

[ntorresd]

This PR introduces parameters plot_constant and x_axis to the visualisation functions of the package. These enable the option to plot constant FOI estimates and their corresponding r-hat values, avoiding ambiguity in the specification of the x-axis by means of x_axis = "time" or x_axis = "age".

Currently, plot_seromodel() behaves differently for constant models. The single FOI estimate and its corresponging r-hat value are displayed in the summary generated by means of plot_summary(), e.g.

data("chagas2012")
seromodel <- fit_seromodel(chagas2012)
plot_seromodel(seromodel, chagas2012, bin_serosurvey = TRUE)

This is preserved as the default behavior, as we would have to choose arbitrarily to plot either with respect to time or age otherwise. Now, if plot_model = TRUE and a constant model is provided, the x-axis can be specified to be either age or time by means of x_axis, e.g.

plot_seromodel(
  seromodel,
  serosurvey,
  plot_constant = TRUE,
  x_axis = "time",
  bin_serosurvey = TRUE
  )

Exceptions for other possible scenarios are handled by the new validation function validate_plot_constant(). This function stops the process whenever plot_constant = TRUE and a non-constant model is provided, or if x_axis is not set up properly when intending to plot constant models. Informative error messages are provided in both cases.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1630c70) to head (5ab5576).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines         1015      1071   +56     
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+ Hits          1015      1071   +56     

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[zmcucunuba]

Excellent! Plot approved.