Ensure model matrices are full-rank

[wlandau]

It is easy to create a rank-deficient model in R:

data(fev_data, package = "mmrm")
formula <- FEV1 ~ ARMCD:AVISIT + RACE
matrix <- model.matrix(object = formula, data = fev_data)
ncol(matrix)
#> [1] 11
as.integer(Matrix::rankMatrix(matrix))
#> [1] 10

As part of #96, I would like to add a new check_rank argument to brm_formula() which checks the rank if TRUE and throws an error for rank-deficient models. I think TRUE is a good default.

For the informative prior scenarios in #96, we can prevent most cases of rank deficiency by dropping nuisance columns until the model matrix is full-rank.

[wlandau]

Fixed in https://github.com/openpharma/brms.mmrm/tree/96