Improve test coverage

[roninsightrx]

Test coverage is intermediate. Especially for the following functions it can be improved:

Running specific tests for package ‘vpc’
Running ‘testthat.R’
vpc Coverage: 68.32%
R/compute_kmmc.R: 0.00%
R/create_vpc_theme.R: 0.00%
R/label_vpc_tte.R: 0.00%
R/read_table_nm.R: 0.00%
R/calc_pred_corr_continuous.R: 25.64%
R/binning.R: 27.87%
R/functions.R: 29.55%
R/pk_functions.R: 33.33%
R/sim_data.R: 40.82%
R/read_vpc.R: 54.29%