Remove unnecessary data from simulated datasets

[ntorresd]

As noted by @ben18785 in #83, the data simulation function generate_sim_data in seroprevalence_data.R adds unnecessary information to the output dataset, namely tsur, test , antibody and survey_label .

This was done for the function extract_seromodel_summary to work for simulated datasets. However, this information shouldn't be mandatory for either simulated nor real datasets, so we need to restructure this function for it to be optional.