This is a utility function to help check input data and covariates provided
smdi_check_covar(data = NULL, covar = NULL)
returns the covariate vector for subsequent tasks or warnings/errors
dataframe or tibble object with partially observed/missing variables
character covariate or covariate vector with partially observed variable/column name(s) to investigate. If NULL, the function automatically looks for and includes all columns with at least one missing observation