Add a when to use the latent model vignette

[seabbs]

As #426 has switched all our public facing examples over to the maringal model we may want a new vignette which deals with when a user might want to use the latent model (likely small data with a lot of custom settings). This would be a helpful section for the primarycensored paper so contributing this would naturally lead to authorship there.

[athowes]

Good idea. Are there data which have a lot of information we might be able to use for this?