Write a more general forecast decomposition function

[nicholasjclark]

plot_mvgam_uncertainty was originally designed to handle models with correlated residuals. It doesn't behave well for State-Space models (where predictors may be used in the process model) and it certainly will fail with dimension-reduced models (i.e. shared latent states). A more general forecast uncertainty decomposition function would be very valuable, but this will take some thought to design and implement