`PointMassFormConstraint` on a `MatrixDirichlet` marginal

[wouterwln]

Whenever we try to put a PointMassFormConstraint on a variable where the marginal is a MatrixDirichlet we run into an error: MWE:

@model function foo(y)
    p ~ MatrixDirichlet(ones(5, 5))
    old_s ~ Categorical([0.2, 0.2, 0.2, 0.2, 0.2])
    new_s ~ Transition(old_s, p)
    y ~ Transition(new_s, diageye(5))
end

constraints = @constraints begin
    q(p, old_s, new_s) = q(p)q(old_s, new_s)
    q(p) :: PointMassFormConstraint()
end

init = @initialization begin
    q(p) = MatrixDirichlet(ones(5, 5))
end

infer(model = foo(), data = (y = [0, 1, 0, 0, 0],), constraints = constraints, initialization = init)

This tries to call mode on the resulting marginal, which is not implemented for a MatrixDirichlet.