Add specialised IID Normal methods for MvNormal

[seabbs]

          I think once we've got all the way down to specifying that its _definitely_ a std norm getting generated `n` times we can use `MvNormal` as a specialism rather than the wider `IID` dispatch.

Or create a method to do this like

generate_latent(IID(D), n) where D <: Normal

?

Originally posted by @SamuelBrand1 in #438 (comment)

[seabbs]

Duplicate of #546