Checks if rd_vcov is a list with elements for all grouping levels or does
not specify a grouping level.
If valid, this function also make sure that rd_vcov is a list per grouping
level by duplicating the contents if necessary.
check_rd_vcov_list(rd_vcov, idvar)A named list per grouping level where each elements contains
information on how the random effects variance-covariance matrices on
that level are structured.
Per level it can be either a character string (e.g. "full") or a
list specifying structures per (groups) of
variable(s) (e.g. list(full = c("a", "b"), indep = "c"))
a character string or a list describing the the random effects variance covariance structure (provided by the user)
vector with the names of all grouping variables (except "lvlone")