Draw samples from a posterior GP/Magma distribution

## Arguments

- pred_gp
A list, typically coming from

`pred_magma`

or`pred_gp`

functions, with argument 'get_full_cov = TRUE'. Required elements:`pred`

,`cov`

.- nb_samples
A number, indicating the number of samples to be drawn from the predictive posterior distribution. For two-dimensional graphs, only one sample can be displayed.