Latent columns

[BaxterEaves]

To enable extra modeling features, such as lace-as-a-prior, we need latent columns, which are basically just dataless columns.

There is a question of implementation: do add a tag to existing columns or treat them as a new column type like

pub enum ColModel {
    Continuous(Column<f64, Gaussian, NormalInvChiSquared, NixHyper>),
    Categorical(Column<u8, Categorical, SymmetricDirichlet, CsdHyper>),
    Count(Column<u32, Poisson, Gamma, PgHyper>),
    MissingNotAtRandom(super::mnar::MissingNotAtRandom),
    Latent(Box<ColModel>), // NEW
}