prepare_covariate_distns: Prepare Covariate Distributions
Description
Resolves missing distributions and parameters by looking at the ALD.
Allows for partial specification (e.g., user specifies one variable,
function auto-detects the rest).
Usage
prepare_covariate_distns(
formula,
ald,
trt_var,
marginal_distns,
marginal_params,
verbose = FALSE
)